Add onGetBlendInfo to GrXferProcessor

Adds an onGetBlendInfo method for GrXferProcessor subclasses to
override instead of overriding getBlendInfo directly. This gives the
base class a chance to initialize the struct with default values
before passing it on. As the BlendInfo struct grows, this will keep
things simple and less error prone.

BUG=skia:
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1049143002
7 files changed