Roll src/third_party/skia/ e18c97b73..56b507925 (4 commits).

https://skia.googlesource.com/skia.git/+log/e18c97b73a03..56b50792531c

$ git log e18c97b73..56b507925 --date=short --no-merges --format='%ad %ae %s'
2016-11-29 mtklein Make gather() look and work more like load().
2016-11-28 mtklein Use /MD for Windows builds.
2016-11-29 mtklein document x_tail encoding
2016-11-29 bsalomon Reland image storage with fixes.

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_trusty_blink_rel
TBR=brianosman@google.com

Review-Url: https://codereview.chromium.org/2536193003
Cr-Commit-Position: refs/heads/master@{#435046}

DEPS

@@ -40,7 +40,7 @@ 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': 'e18c97b73a0392b2eee57a111122dd5b637e36e6',
+  'skia_revision': '56b50792531c956be3e0c472b098255c62c7e755',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.