[webp] Make createColorTransform bool returning

Make createColorTransform indicate success to callers via a bool
return to support dynamic color correction issue 353818.

No behavioral change, covered by existing layout / unit tests.

BUG=404544

Review URL: https://codereview.chromium.org/577833004

git-svn-id: svn://svn.chromium.org/blink/trunk@182211 bbb929c8-8fbe-4397-9dbb-9b2b20218538