Add synonyms for collection types over MutationObserver-related types.

With r172148, the MutationObserver collection types got even bulkier.
Tidy up and shorten by adding some typedefs.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/257513002

git-svn-id: svn://svn.chromium.org/blink/trunk@172475 bbb929c8-8fbe-4397-9dbb-9b2b20218538