Convert storage API SettingsFrontend to ProfileKeyedAPI

* Convert SettingsFrontend to use BrowserContext instead of Profile
* Convert SettingsFrontend to a ProfileKeyedAPI, which is-a BrowserContextKeyedService
* Remove SettingsFrontend from ExtensionService

This allows the storage API to be used from app_shell, which uses BrowserContext and has no ExtensionService.

BUG=346871
TEST=existing browser_tests and unit_tests of StorageAPI
R=kalman@chromium.org, rdevlin.cronin@chromium.org
TBR=zea@chromium.org for mechanical change to chrome/browser/sync

Review URL: https://codereview.chromium.org/180253008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253931 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253931