ServiceWorker: Expose Cache/CacheStorage interfaces, fix get() response

Per spec [1], in the Service Worker global context, Cache and
CacheStorage should be exposed to scripts, so remove
[NoInterfaceObject]. Also, a CacheStorage get() call that doesn't
find anything should resolve to undefined, rather than false.

Discovered via pending tests which should land soon.

BUG=392621
TEST=http://crrev.com/430993002
R=gavinp@chromium.org,michaeln@chromium.org

Review URL: https://codereview.chromium.org/477133007

git-svn-id: svn://svn.chromium.org/blink/trunk@180793 bbb929c8-8fbe-4397-9dbb-9b2b20218538