Don't use strong enums when compiling with Clang on Windows

They are exposed at the ABI boundary (some functions take strong enums, and that goes
into the mangling of the function name). Since we cannot yet compile all of Blink
with Clang on Windows, let's turn this feature off for now.

BUG=82385

Review URL: https://codereview.chromium.org/142083007

git-svn-id: svn://svn.chromium.org/blink/trunk@166332 bbb929c8-8fbe-4397-9dbb-9b2b20218538