Gtk3: Use Chrome's border width instead of GTK's for frame button layout

Previously, a GtkHeaderBar's border was used for laying out frame
buttons in the tabstrip.  However, we never draw a GTK border --
instead, Chrome draws its own 2dp border on all edges.  This could
lead to button placement being off by 1dp when the gtk border was only
1dp or 0dp wide.

Also, small changes in layout code:
* Apply the "image-button" class to the avatar button.
* Make avatar button layout code more consistent with other buttons.

BUG=753067
R=erg@chromium.org,sky@chromium.org

Change-Id: I017c4e77892a8a244fb0772ff5432e9b8ef012bf
Reviewed-on: https://chromium-review.googlesource.com/759430
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Elliot Glaysher <erg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515283}
6 files changed