Cleanup: Turn GrReducedClip into a class with a static function.

Clean up namespace usage.

Similar to what was done in
https://skia.googlesource.com/skia/+/a5414c4a8efc3119ee20fcee96c0bf68a04909c7

BUG=None
TEST=None
R=bsalomon@google.com

Review URL: https://codereview.chromium.org/653393003