Increase default alpha tolerance in ImageAsyncReadPixels

Change-Id: I4165c550529ddb1bd998e9a9a21149e4f1bf5264
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/745060
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed