Use CSS.supports instead of '..' in ....style

document.documentElement.style is null in some XML documents.
The previous snippet caused the following error:

Uncaught TypeError: Cannot use 'in' operator to search for 'animation' in null

To fix this bug, `'animation' in document.documentElement.style` has been
replaced with `CSS.supports('animation', '9s')`. This method was introduced
in Chromium 28, but it is not necessary to detect whether this method is
supported because the required createShadowRoot method for embeds is not
available in Chromium 32 and earlier.

extensions/chromium/contentscript.js

@@ -15,7 +15,7 @@ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and
 limitations under the License.
 */
-/* globals chrome */
+/* globals chrome, CSS */
 
 'use strict';
 
@@ -45,7 +45,7 @@ if (typeof Element.prototype.createShadowRoot !== 'undefined') {
 
 // Only observe the document if we can make use of Shadow DOM.
 if (createShadowRoot) {
-  if ('animation' in document.documentElement.style) {
+  if (CSS.supports('animation', '0s')) {
     document.addEventListener('animationstart', onAnimationStart, true);
   } else {
     document.addEventListener('webkitAnimationStart', onAnimationStart, true);