Oilpan: enable appcache + move DocumentLoader to the heap.

      DocumentLoader belongs on the heap. References to it
are kept by an object that is on the heap (FrameLoader),
and DocumentLoader itself has heap references.

Along with moving DocumentLoader, have the two appcache
objects (ApplicationCache and ApplicationCacheHost) be
under Oilpan's control always.

R=haraken
BUG=340522,497595

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197576

Review URL: https://codereview.chromium.org/1194003004

git-svn-id: svn://svn.chromium.org/blink/trunk@197664 bbb929c8-8fbe-4397-9dbb-9b2b20218538