Fix type annotation to pass the closure compiler.

Trivial fix to make the closure's Buildbot green.
http://build.chromium.org/p/chromium.fyi/builders/Closure%20Compilation%20Linux

BUG=none
TEST=GYP_GENERATORS=ninja tools/gyp/gyp --depth . ui/file_manager/file_manager/foreground/js/compiled_resources.gyp && ninja -C out/Default
TBR=yawano@chromium.org

Review URL: https://codereview.chromium.org/800393003

Cr-Commit-Position: refs/heads/master@{#308339}

ui/file_manager/file_manager/foreground/js/metadata/metadata_cache.js

@@ -129,7 +129,7 @@ MetadataCache.createFull = function(volumeManager) {
  * Clones metadata entry. Metadata entries may contain scalars, arrays,
  * hash arrays and Date object. Other objects are not supported.
  * @param {Object} metadata Metadata object.
- * @return {!Object} Cloned entry.
+ * @return {Object} Cloned entry.
  */
 MetadataCache.cloneMetadata = function(metadata) {
   if (metadata instanceof Array) {