Favor full screen clears for platforms that desire initialized memory

In the cases this touches, although we may provably never touch the un-initialized pixels, this may appease the security checks.

Change-Id: I06c87b127db6acf6bc8bb84590d6f93b02f8328d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206261
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>