remove SkCanvas::kIsOpaque_SaveLayerFlag

This is a performance-only hint that no one but fuzzers
is using.  It's even explicitly filtered out in Android.

The fuzzers have noticed they can trick us into allocating
uninitialized memory and treating it as opaque, blending
uninitialized pixels, etc.

Since no one's using this, we can just kill the bit.

Bug: skia:7566, chromium:808830

Change-Id: Id74a85e51bc4d0907b4127eb9ac5b02576f8f0a7
Reviewed-on: https://skia-review.googlesource.com/104441
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
8 files changed