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View's native theme.

This fixes a few bugs where we weren't using the right native theme,
whether by not updating after the NativeTheme changes or by trying to
access the NativeTheme before the View is added to a hierarchy (which
yields the default NativeTheme --- for most platforms, this didn't
effectively create a bug as there's only one NativeTheme).

Best example is that now the sad tab respects the GTK native theme.

Get rid of Ash's tray_constants::kBackgroundColor in favor of using the
bubble background color from the NativeTheme.

This is a reland of 097f9cde with fix
for asan failure.

BUG=711183, 693282
TBR=stevenjb@chromium.org,tdanderson@chromium.org

Review-Url: https://codereview.chromium.org/2838273002
Cr-Commit-Position: refs/heads/master@{#468157}