Split ExtensionSystem interface from ExtensionSystemImpl implementation, part 1.

This moves ExtensionSystem to extensions/browser.
It also moves setting the channel for Features to ChromeExtensionsBrowserClient.
It also removes the duplicate ExtensionSystem::GetForBrowserContext.

BUG=337707
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/147923005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247788 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247788