Quota: Return a pair of usage and granted_quota on requesting quota.

This adds "usage" param into the reply message of the request quota operation.
That is necessary to implement new Quota Management API [1] behind a flag.

[1] http://www.w3.org/TR/2013/WD-quota-api-20131105/


BUG=332325
TEST=browser_tests

Review URL: https://codereview.chromium.org/142903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245969 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245969