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..bd3a2ea23090cfeddcf11d13a35c49f4ad242aed 100644
--- a/ios/net/crn_http_protocol_handler_proxy.h
+++ b/ios/net/crn_http_protocol_handler_proxy.h
@@ -13,11 +13,19 @@
 // NSURLProtocol and its client.
 @protocol CRNHTTPProtocolHandlerProxy<CRNNetworkClientProtocol>
 
+// All of the methods defined below must be called on the client thread. Methods
+// defined by |CRNNetworkClientProtocol| can be called on any thread.
+
 // Invalidates any reference to the protocol handler: the handler will never
 // be called after this.
-// Called from the client thread.
 - (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_