Roll src/third_party/WebKit 531baa3:31cf1e7 (svn 189321:189323)

Summary of changes available at:
https://chromium.googlesource.com/chromium/blink/+log/531baa3..31cf1e7

TBR=vsevik@chromium.org,japhet@chromium.org

Review URL: https://codereview.chromium.org/875143004

Cr-Commit-Position: refs/heads/master@{#314128}

DEPS

@@ -34,7 +34,7 @@ vars = {
   'llvm_url': 'http://src.chromium.org/llvm-project',
   'llvm_git': 'https://llvm.googlesource.com',
   'webkit_trunk': 'http://src.chromium.org/blink/trunk',
-  'webkit_revision': '531baa3ccf89f9934ae65ea757ae216ff0abe0c4', # from svn revision 189321
+  'webkit_revision': '31cf1e740411255429b57d189095df90eb435565', # from svn revision 189323
   'chromium_git': 'https://chromium.googlesource.com',
   'chromiumos_git': 'https://chromium.googlesource.com/chromiumos',
   'pdfium_git': 'https://pdfium.googlesource.com',