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This CL also splits out the code wrt to handling the GTK menu for the app indicator icon out of app_indicator_icon.cc in order to share code with gtk2_status_icon.cc

BUG=267195
TEST=Manual, see steps below.
1) Run chrome in a system which does not have libappindicator
2) Generate HTML 5 notifications via https://developer.cdn.mozilla.net/media/uploads/demos/e/l/elfoxero/c17223c414d8ddafb7808972b5617d9e/html5-notifications_1400214081_demo_package/index.html
3) Ensure that the bell icon shows up in the system tray and that clicking on the bell icon brings up the message center

Review URL: https://codereview.chromium.org/306063003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273999 0039d316-1c4b-4281-b951-d872f2087c98