simplify setMetricsAndImage API

Just return the amount allocated during the merge.
Add TODO to simplify when I'm sure there are no
collisions during the merge.

Change-Id: Ieb568c4096fc547c54303689b232c68d78f3f36b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/317382
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed