Make WebContentsDelegate::OpenColorChooser return NULL on failure

      Changing WebContentsDelegate::OpenColorChooser to return NULL on failure so we don't put the same ColorChooser into two scoped_ptrs(WebContentsImpl::color_chooser_)

BUG=331790

Review URL: https://codereview.chromium.org/128053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244710 0039d316-1c4b-4281-b951-d872f2087c98