Adds associated interface support between RenderFrameHost and RenderFrame.

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.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2310563002
Cr-Commit-Position: refs/heads/master@{#418717}
diff --git a/ipc/ipc_channel_proxy.h b/ipc/ipc_channel_proxy.h
index 06b32671..14b5fe0 100644
--- a/ipc/ipc_channel_proxy.h
+++ b/ipc/ipc_channel_proxy.h
@@ -281,6 +281,9 @@
     bool OnMessageReceived(const Message& message) override;
     void OnChannelConnected(int32_t peer_pid) override;
     void OnChannelError() override;
+    void OnAssociatedInterfaceRequest(
+        const std::string& interface_name,
+        mojo::ScopedInterfaceEndpointHandle handle) override;
 
     // Like OnMessageReceived but doesn't try the filters.
     bool OnMessageReceivedNoFilter(const Message& message);
@@ -325,6 +328,9 @@
     void OnDispatchConnected();
     void OnDispatchError();
     void OnDispatchBadMessage(const Message& message);
+    void OnDispatchAssociatedInterfaceRequest(
+        const std::string& interface_name,
+        mojo::ScopedInterfaceEndpointHandle handle);
 
     void SendFromThisThread(Message* message);
     void ClearChannel();