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With this CL, IBusEngineService can handle MethodCall and Signal emitting.
To avoid CL delta bloat, let me split the CL.
I will send CL for unit tests soon.

BUG=126947
TEST=try bots

Review URL: https://chromiumcodereview.appspot.com/10806006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148038 0039d316-1c4b-4281-b951-d872f2087c98