Fix return type of Benchmark::getSize

This was confusing, for no good reason.

Change-Id: Ia0a87ab1dba7f4d42a8b2cba0d0d5c69914f7e39
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/712937
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
28 files changed