mlzboy/manifest.json

## manifest.json
{
  "name": "SyncExtension",
  "version": "0.1",
  "manifest_version": 2,

  "description": "Storage Sync Extension",

  "permissions": [ "storage" ],

  "browser_action": {
    "default_popup": "popup.html"
  }
}

## popup.html
<!DOCTYPE html>
<html lang="ja">
<head>
  <meta charset="utf-8" />
</head>
<body>
  <div id="data"></div>
  <input type="text" id="text"></input>
  <button id="set">Set</button>
  <script src="popup.js"></script>
</body>
</html>

## popup.js

// popup.js

document.body.onload = function() {
  chrome.storage.sync.get("data", function(items) {
    if (!chrome.runtime.error) {
      console.log(items);
      document.getElementById("data").innerText = items.data;
    }
  });
}

document.getElementById("set").onclick = function() {
  var d = document.getElementById("text").value;
  chrome.storage.sync.set({ "data" : d }, function() {
    if (chrome.runtime.error) {
      console.log("Runtime error.");
    }
  });
  window.close();
}
	{
	"name": "SyncExtension",
	"version": "0.1",
	"manifest_version": 2,

	"description": "Storage Sync Extension",

	"permissions": [ "storage" ],

	"browser_action": {
	"default_popup": "popup.html"
	}
	}