fixup! Implement Chromium Theme for Tizen.

fixup! Implement Chromium Theme for Tizen.

Since https://codereview.chromium.org/738113003,
LayoutTheme::theme method is not defined in LayoutThemeChromiumDefault
anymore. Instead, each subclass (Win,Android,Mac and Tizen)
that needs a implementation of ::theme adds its own.

Following that, Efl/Tizen port can now have its own
implementation of LayoutTheme::theme without the
RenderThemeChromiumDefault_override hack.

Patch removes the hack, accordingly.
Reviewed by: Balazs Kelemen, SeungSeop Park, arno renevier

Change-Id: I5cdaa53abe0d39d8afd701361337dbcda29ec936
Signed-off-by: Antonio Gomes <a1.gomes@samsung.com>