Roll src/third_party/WebKit 4a137c2:20a3bc7 (svn 183430:183520)

Summary of changes available at:
https://chromium.googlesource.com/chromium/blink/+log/4a137c2..20a3bc7

TBR=eseidel@chromium.org,caseq@chromium.org,apavlov@chromium.org

Review URL: https://codereview.chromium.org/643843002

Cr-Commit-Position: refs/heads/master@{#299085}

DEPS

@@ -36,7 +36,7 @@ vars = {
   'libcxx_revision': '48198f9110397fff47fe7c37cbfa296be7d44d3d',
   'libcxxabi_revision': '4ad1009ab3a59fa7a6896d74d5e4de5885697f95',
   'webkit_trunk': 'http://src.chromium.org/blink/trunk',
-  'webkit_revision': '4a137c27d133243ce3cf9530fe2f6b328bc66c38', # from svn revision 183430
+  'webkit_revision': '20a3bc73c6aeac3f5846faa54b166501a1909d43', # from svn revision 183520
   'chromium_git': 'https://chromium.googlesource.com',
   'chromiumos_git': 'https://chromium.googlesource.com/chromiumos',
   'pdfium_git': 'https://pdfium.googlesource.com',