Define Color::transparent et al. when using Clang on Windows

MSVC treats their in-class initializations as definitions, but other
compilers don't. This updates the #ifdef to handle the Clang on Windows
case.

BUG=82385
TEST=build blink_web.dll with Clang on Windows in Release mode

Review URL: https://codereview.chromium.org/330823006

git-svn-id: svn://svn.chromium.org/blink/trunk@176469 bbb929c8-8fbe-4397-9dbb-9b2b20218538