Introduces a new content::AssociatedInterfaceRegistry and
content::AssociatedInterfaceProvider to serve as Channel-associated
interface counterparts to their shell namesakes.

RenderFrameHost and RenderFrame each have both of these things on them,
and they're routed to the other side using a new RouteProvider interface
buried in RenderProcessHostImpl / ChildThreadImpl on either end.

Also adds a WebContentsInterfaceRegistry to every WebContents,
and a helper WebContentsFrameBindingSet<T> to allow consumers to
very easily manage the lifetime of associated interface bindings
for each frame in a WebContents.

See the follow-up patch for example usage.

Part a series of CLs to enable and demonstrate WebContents associated
interfaces:

  1. https://codereview.chromium.org/2309513002
  2. https://codereview.chromium.org/2316963005
  3. This CL
  4. https://codereview.chromium.org/2310583002

BUG=612500
CQ_INCLUDE_TRYBOTS=master.tryserver...