Initial gtk3 support

Currently we're sticking with deprecated apis to make it easier to support both gtk2 and gtk3. If Chrome ever drops gtk2 support, it might be a good idea to do a complete port.

BUG=132847
R=erg@chromium.org,estade@chromium.org

Review URL: https://codereview.chromium.org/1234223005

Cr-Commit-Position: refs/heads/master@{#348339}
18 files changed