Make SkBitmap/SkPixmap::erase* do so in sRGB

We generally consider untagged colors to be sRGB, so this makes us more
consistent with other parts of the API.

Add a test.

Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Change-Id: I5468c86ad92164797a65ffd9fbe471e01a97a2ca
Reviewed-on: https://skia-review.googlesource.com/c/179245
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed