move SkProcCoeffXfermode unflatten constructor into .cpp, and range-check its mode

BUG=

Review URL: https://codereview.chromium.org/79543009

git-svn-id: http://skia.googlecode.com/svn/trunk@12355 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed