Roll external/skia 311cc6b39..ae7d3f399 (3 commits)

https://skia.googlesource.com/skia.git/+log/311cc6b39..ae7d3f399

2017-09-21 robertphillips@google.com Add native caching of uniquely keyed GrTextureProxies (take 2)
2017-09-21 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ a5dc625c9..61491505e (1 commit)
2017-09-21 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ c8bee3351..a5dc625c9 (1 commit)

Test: Presubmit checks will test this change.
Change-Id: I23228d3550911a34f13bd647ebebb16bd5ba417a