CrNet: add pauseable NSURLProtocol and switch to using it

ios/net/crn_http_protocol_handler_proxy.h

Index: ios/net/crn_http_protocol_handler_proxy.h
diff --git a/ios/net/crn_http_protocol_handler_proxy.h b/ios/net/crn_http_protocol_handler_proxy.h
index 898db4ad448c1a170a70008273f9b85cbdd96e34..72d762c85f48e796c3c1b7a7a61b0120a9df4c7c 100644
--- a/ios/net/crn_http_protocol_handler_proxy.h
+++ b/ios/net/crn_http_protocol_handler_proxy.h
@@ -18,6 +18,12 @@
 // Called from the client thread.

[droger, 2015/06/11 08:56:19]

Maybe remove this "Called from the client thread." and add a comment for the
whole class saying that:
- methods inherited from CRNNetworkClientProtocol can be called from any thread
(but all on the same)
- the other methods must be called on the client thread.

[droger, 2015/06/11 08:58:36]

Actually I'm not sure if all the methods of CRNNetworkClientProtocol have to be
called from the same thread or not. For sure there aren't any checks that they
indeed are.

[droger, 2015/06/11 09:01:21]

Looking at the code I would think that in practice they are all called from the
same thread, and other CRNNetworkClientProtocol have DCHECKs for that (see for
example CRNForwardingNetworkClient).
Digging into this may be out of scope of your work though.

[Elly Fong-Jones, 2015/06/11 13:49:33]

Documented, at least.

 - (void)invalidate;
 
+// Pauses notifications from this protocol handler.
+- (void)pause;
+
+// Resumes notifications from this protocol handler.
+- (void)resume;
+
 @end
 
 #endif  // IOS_NET_CRN_HTTP_PROTOCOL_HANDLER_PROXY_H_