fixup! Introduce blink::mojom:::RemoteMainFrame interface

This CL complements https://crrev.com/c/2307152, where
blink::mojom::RemoteMainFrame was introduced.

The missing bit was the plumbing in
RenderFrameProxy::OnAssociatedInterfaceRequest(), where the mojo
interface is registered.

BUG=1102440
R=haraken@chromium.org

Change-Id: I47567ca0f865e3868bb32f726b1fc09c738900ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2323136
Auto-Submit: Antonio Gomes (GMT-4) <tonikitoo@igalia.com>
Commit-Queue: Antonio Gomes (GMT-4) <tonikitoo@igalia.com>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792334}

content/renderer/render_frame_proxy.cc

@@ -413,6 +413,8 @@ void RenderFrameProxy::OnAssociatedInterfaceRequest(
     mojo::ScopedInterfaceEndpointHandle handle) {
   if (interface_name == blink::mojom::RemoteFrame::Name_) {
     associated_interfaces_.TryBindInterface(interface_name, &handle);
+  } else if (interface_name == blink::mojom::RemoteMainFrame::Name_) {
+    associated_interfaces_.TryBindInterface(interface_name, &handle);
   } else if (interface_name == content::mojom::RenderFrameProxy::Name_) {
     render_frame_proxy_receiver_.Bind(
         mojo::PendingAssociatedReceiver<mojom::RenderFrameProxy>(