SOLUTION FOR: DEPRECATED >> Uri downloadUrl = taskSnapshot.getDownloadUrl

MainActivity.java

// Udacity - Does not working
@Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == RC_SIGN_IN) {
            if (resultCode == RESULT_OK) {
                Toast.makeText(getContext(), "Signed in!", Toast.LENGTH_SHORT).show();
            } else if (resultCode == RESULT_CANCELED) {
                Toast.makeText(getContext(), "Sign in canceled", Toast.LENGTH_SHORT).show();
                getActivity().finish();
            } else if (requestCode == RC_PHOTO_PICKER && resultCode == RESULT_OK) {
                Uri selectedImageUri = data.getData();
                // Get a reference to store file at chat_photos/<FILENAME>
                StorageReference photoRef = mChatPhotosStorageReference.child(selectedImageUri.getLastPathSegment());
                mChatPhotosStorageReference.child(selectedImageUri.getLastPathSegment());

                // Upload file to Firebase Storage
                photoRef.putFile(selectedImageUri)
                        .addOnSuccessListener(this, new OnSuccessListener<UploadTask.TaskSnapshot>() {
                            public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                                // When the image has successfully uploaded, we get its download URL
                                Uri downloadUrl = taskSnapshot.getDownloadUrl();

                                // Set the download URL to the message box, so that the user can send it to the database
                                ChatMessageClass friendlyMessage = new ChatMessageClass(null, mUsername, downloadUrl.toString());
                                mMessagesDatabaseReference.push().setValue(friendlyMessage);
                            }
                        });
            }
        }
    }

// ------------------------------------------------------------------------------------------------------------------------------------

//Update - SOLUTION
 public void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == RC_SIGN_IN && resultCode == RESULT_OK) {
            Toast.makeText(this, "Signed in!", Toast.LENGTH_SHORT).show();
        } else if (resultCode == RESULT_CANCELED) {
            Toast.makeText(this, "Sign in canceled!", Toast.LENGTH_SHORT).show();
        } else if (requestCode == RC_PHOTO_PICKER && resultCode == RESULT_OK) {
            Uri selectedImageUri = data.getData();

            // Get a reference to store file at chat_photos/<FILENAME>
            final StorageReference photoRef = mChatPhotosStorageReference.child(selectedImageUri.getLastPathSegment());

            photoRef.putFile(selectedImageUri)
                    .addOnSuccessListener(this, new OnSuccessListener<UploadTask.TaskSnapshot>() {
                        @Override
                        public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                            //When the image has successfully uploaded, get its download URL
                            photoRef.getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
                                @Override
                                public void onSuccess(Uri uri) {
                                    Uri dlUri = uri;
                                    FriendlyMessage friendlyMessage = new FriendlyMessage(null, mUsername, dlUri.toString());
                                    mMessagesDatabaseReference.push().setValue(friendlyMessage);
                                }
                            });
                        }
                    });
        }
    }