Fix crash in webkit_unit_tests on Windows.

Sometimes the theme engine is null (commonly,
RenderTableCellDeathTest.CanSetColumn crashes in
WebCore::ScrollbarThemeAuraOrGtk::scrollbarThickness)
because we're skipping over the implementation of themeEngine()
in WebKitPlatformSupportChildImpl.

BUG=None
R=scottmg@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/25040004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226250 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226250