Roll src/third_party/skia 9b4b91f5:4b91f76

Summary of changes available at:
https://chromium.googlesource.com/skia/+log/9b4b91f5..4b91f76

CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
TBR=bungeman@google.com

Review URL: https://codereview.chromium.org/1130763009

Cr-Commit-Position: refs/heads/master@{#330560}

DEPS

@@ -42,7 +42,7 @@ vars = {
   'boringssl_git': 'https://boringssl.googlesource.com',
   'libvpx_revision': 'd1c022c097f22987d521643c4802b623e572393a',
   'sfntly_revision': '1bdaae8fc788a5ac8936d68bf24f37d977a13dac',
-  'skia_revision': '9b4b91f5c222da584bb72d5880f654ae0081454d',
+  'skia_revision': '4b91f768b348aa1cebeb54f3ff9331938734c242',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling Skia
   # and V8 without interference from each other.