[graphite] Guard removal of entries from the ProxyCache

Not quite sure how we're getting into this situation but it appears that a key that has been invalidated is already no longer in the cache when we get to processing the invalidation messages.

Bug: chromium:1480570
Change-Id: I7f1eaf29b2e79549a37b7f786be1792d8b400bda
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/753620
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed