Add Make functions for runtime blends.

This does basic plumbing work, but lacks a real implementation for the
custom blend subclass. That subclass is added in the followup CL of this
CL chain.

Change-Id: I8db0eb42737a739b49741ffd6f4a283a2c0a9d62
Bug: skia:12080
Auto-Submit: John Stiles <>
Reviewed-by: Brian Osman <>
Commit-Queue: John Stiles <>
3 files changed