Move storage API interface description to src/extensions

The extensions local-storage API is moving into src/extensions so it can be
used by app_shell. This finishes that move.

* Move storage.json to extensions/common/api
* Move storage functions into core_api namespace
* Remove ExtensionService references from StorageApiUnittest

BUG=348058
TEST=unit_tests *Storage* and browser_tests *Storage*

Review URL: https://codereview.chromium.org/197883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256901 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256901