Remove GTK2 code

GTK2 code was historically kept around to keep the electron build working.  But
as of Nov 26, 2017, electron now uses GTK3 [1], so it should now be safe to
remove all of the old GTK2-only code.

[1] https://github.com/electron/electron/issues/2927#issuecomment-347092515

BUG=876558
R=sky

Change-Id: I78402053ae508ccd8fc7ac73697a861bb1ebacf6
Reviewed-on: https://chromium-review.googlesource.com/1183858
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#585265}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 287339e07364162daa8fd6f90ef1d2338f32bed5
4 files changed