DevTools: Better fix for a null pointer exception in RendererOverridesHandler::ParseCaptureParameters

BUG=283782

Review URL: https://codereview.chromium.org/34633003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230505 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230505