Roll src/third_party/skia/ a9f64dec6..4e56a9139 (2 commits).

https://chromium.googlesource.com/skia.git/+log/a9f64dec63b6..4e56a9139373

$ git log a9f64dec6..4e56a9139 --date=short --no-merges --format='%ad %ae %s'
2016-10-21 jvanverth Add Android viewer to GN
2016-10-21 reed add pipe to nanobench

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, see:
http://www.chromium.org/developers/tree-sheriffs/sheriff-details-chromium#TOC-Failures-due-to-DEPS-rolls

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel
TBR=mtklein@google.com

Review-Url: https://chromiumcodereview.appspot.com/2436343002
Cr-Commit-Position: refs/heads/master@{#426810}

DEPS

@@ -39,7 +39,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling Skia
   # and whatever else without interference from each other.
-  'skia_revision': 'a9f64dec63b6f4169ba16f8b6c63fff5a6494029',
+  'skia_revision': '4e56a91393733359ba19967a78df26184e53854e',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.