https://www.chromium.org/developers/how-tos/install-depot-tools
Opera Developer 35.0.2052.0 -> http://commondatastorage.googleapis.com/chromium-browser-official/chromium-48.0.2547.0.tar.xz
Opera Beta 34.0.2036.3 -> http://commondatastorage.googleapis.com/chromium-browser-official/chromium-47.0.2526.35.tar.xz
Opera 33.0.1990.115 -> http://commondatastorage.googleapis.com/chromium-browser-official/chromium-46.0.2490.86.tar.xz
tar -xf chromium-<version>.tar.xz
cd chromium-<version>
# Visit https://chromium.googlesource.com/chromium/src/+/master/docs/linux_build_instructions_prerequisites.md
./build/install-build-deps.sh --no-arm
./build/gyp_chromium -Dcomponent=shared_library -Dffmpeg_branding=Chrome -Dffmpeg_soname_version=<opera_version>
# opera_version = 35 is for Opera Developer, 34 for Opera Beta and 33 for Opera
ninja -C out/Release ffmpeg
# Declare OPERADIR to /usr/lib/x86_64-linux-gnu/opera-developer,/usr/lib/x86_64-linux-gnu/opera-beta or /usr/lib/x86_64-linux-gnu/opera
sudo mkdir $OPERADIR/lib_extra
cp out/Release/lib/libffmpeg.so* $OPERADIR/lib_extra
FYI, had to comment out a line to be able to build Chromium 46 for Opera developer 33: https://code.google.com/p/chromium/issues/detail?id=515917, otherwise this still worked fine.