Disabling flaky test PluginTest.DeletePluginInDeallocate

http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&testType=content_browsertests&tests=PluginTest.DeletePluginInDeallocate

BUG=310336
TBR=jackhou

Review URL: https://codereview.chromium.org/39223004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230636 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230636