Roll src/third_party/skia/ 1638c0dee..b5acf6e70 (4 commits).

https://chromium.googlesource.com/skia.git/+log/1638c0deea80..b5acf6e702e3

$ git log 1638c0dee..b5acf6e70 --date=short --no-merges --format='%ad %ae %s'
2016-07-25 mtklein Add a clamp stage to SkRasterPipelineBlitter.
2016-07-25 caryclark fix fuzz
2016-07-25 mtklein Arithmetic xfermode stage.
2016-07-25 borenet webpages_playback: Make --upload_to_partner_bucket work without --upload

BUG=630736

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_blink_rel
TBR=tomhudson@google.com

Review-Url: https://codereview.chromium.org/2177883002
Cr-Commit-Position: refs/heads/master@{#407470}

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': '1638c0deea80e4a580fe43babeb04279dd163e98',
+  'skia_revision': 'b5acf6e702e318b405a04d38bfdac602dc3ed773',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling V8
   # and whatever else without interference from each other.