Remove GM::onDrawBackground

Bug: skia:8731
Change-Id: Ied8f3035eff1c37dd4b785f850f95a8c56a8a67b
Reviewed-on: https://skia-review.googlesource.com/c/188632
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
4 files changed