Give GrSimpleMeshDrawOpHelper a cpp file

Change-Id: I1ac3f7d41d3d546801a7fc14008b801a11429472
Reviewed-on: https://skia-review.googlesource.com/22368
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
3 files changed