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When complete, the Gtk3 theming code will look completely different
from the Gtk2 code.  This CL splits out NativeThemeGtk in preparation
of this change.

The planned change for gtk3 is to use foreign drawing (see
gtk/demos/gtk-demo/foreigndrawing.c) to provide an implementation of
NativeTheme::GetPartSize and NativeTheme::Paint*.

The Gtk2 theme will continue to use the NativeThemeBase
implementations of these functions while just supplying basic colors.

BUG=132847
R=erg@chromium.org

Review-Url: https://codereview.chromium.org/2579683002
Cr-Commit-Position: refs/heads/master@{#439027}