Moving gRPC support interfaces out of cronet and into a new component.

net/url_request/url_request_context.h

Index: net/url_request/url_request_context.h
diff --git a/net/url_request/url_request_context.h b/net/url_request/url_request_context.h
index e483178049b907426506123334e5adeb34c5a017..3014dbf6031fe494a5b47e083b35dcf99e3a2088 100644
--- a/net/url_request/url_request_context.h
+++ b/net/url_request/url_request_context.h
@@ -238,6 +238,8 @@ class NET_EXPORT URLRequestContext
     return enable_referrer_policy_header_;
   }
 
+  void DetachFromThread();
+
  private:
   // ---------------------------------------------------------------------------
   // Important: When adding any new members below, consider whether they need to