Pull GN r249300

This is to pick up some threading enhancements.

TBR=scottmg

Review URL: https://codereview.chromium.org/151323004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249384 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249384
Disable PluginTest.SelfDeletePluginGetUrl, PluginTest.SelfDeletePluginInvoke, PluginTest.SelfDeletePluginInvokeAlert
under OS X.

BUG=340806
TBR=tfarina@chromium.org

Review URL: https://codereview.chromium.org/153173005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249385 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249385