Roll src/third_party/skia/ 615af784a..7bffcd267 (4 commits).

https://chromium.googlesource.com/skia.git/+log/615af784ab6e..7bffcd267301

$ git log 615af784a..7bffcd267 --date=short --no-merges --format='%ad %ae %s'
2016-09-15 bsalomon Fix key computation for GrPaths
2016-09-15 mtklein GN: port Valgrind bots.
2016-09-15 bsalomon Add GrContextOptions flag to disable distance field path renderer.
2016-09-15 robertphillips Fix color bug in SkRRectsGaussianEdgeShader

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

Review-Url: https://codereview.chromium.org/2348623002
Cr-Commit-Position: refs/heads/master@{#419002}

DEPS

@@ -36,7 +36,7 @@ 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': '615af784ab6ee0a1394b0ce62a49fce7be157451',
+  'skia_revision': '7bffcd2673015f04fd3e20785746be38dd81b566',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.