Files.app: Fix the argument type of metadata observers.

FileWatcher expects a map of entry URL and metadata as an third argument of observers, but MetadataCache passes array of metadata.
This make FileTable fail metadata update, and it causes flakiness of tests.

BUG=330549, 316918
TEST=browser_tests

Review URL: https://codereview.chromium.org/120903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242475 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242475