Gtk3: Refactor GtkButtonImageSource::GetImageForScale

This CL refactors GetImageForScale() on Gtk3 so no widget gets created.  Only
a GtkStyleContext is needed.

In addition, it adds GtkStateFlagFocused to the style context if |focus_| is true.
This fixes focused button rendering on Ambiance, which doesn't use a focus
rectangle, but instead changes the button's border directly.


Cr-Commit-Position: refs/heads/master@{#453292}
4 files changed