Fix SkImageFilterCache race

BUG=chromium:668937
R=reed@google.com,robertphillips@google.com

Change-Id: I72ceaf1d88946c277a38f32014640e01ae9cbded
Reviewed-on: https://skia-review.googlesource.com/6954
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed