move Coeff into blendmode

Bug: skia:
Change-Id: Ib4f3398b0cdc6b0f575f6db80d2ce0bf9666f077
Reviewed-on: https://skia-review.googlesource.com/18843
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
11 files changed