Fix inconsistent title bar theme usage when GTK theme is set

This CL ensures we consistently use GTK theme colours for PWA windows.
Before this change the title bar colour would use the GTK theme but
the title text would not; resulting in unreadable text for dark GTK themes.

This CL adds BrowserFrame::ShouldIgnoreTheme() as an equivalent to
BrowserNonClientFrameView::ShouldPaintAsThemed(). The latter cannot be
used everywhere due to initialisation order dependencies.
Deduping these functions is done in a separate CL
https://chromium-review.googlesource.com/c/chromium/src/+/1453217
in the interest of keeping this CL merge friendly.

Before: https://bugs.chromium.org/p/chromium/issues/attachment?aid=378056&signed_aid=KLS7LkKg4N2vijhbrVDErQ==&inline=1
After: https://bugs.chromium.org/p/chromium/issues/attachment?aid=378057&signed_aid=_AbZIHu6j7Hwm57_i9KhRA==&inline=1

Bug: 927381
Change-Id: I308d8c3c0606cb54ba708cee1ec89a620c1f9d6c
Reviewed-on: https://chromium-review.googlesource.com/c/1453558
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629488}
4 files changed