remove stray weakref include

Change-Id: I016de62543b5ba16a7193262cea343a77a71ba3b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/304201
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed