Don't call RemoteMainFrameHost::RouteCloseEvent() on unbound remote

It seems possible that a race condition to happen between WebViewImpl's
CloseWindowSoon() schedules a call to DoDeferredCloseWindowSoon() and
that method actually being called: even though WebViewImpl will be
alive (otherwise the weak pointer would have prevented that callback
from being called), WebViewImpl::DidDetachRemoteMainFrame() might have
been called in between those two points, unbinding the remote and
causing the issue reported as part of crbug.com/1127329.

Bug: 1127329
Change-Id: I189fc1b38f315dc420d97eda5b5c01d346db8033
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2409499Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Cr-Commit-Position: refs/heads/master@{#806730}

third_party/blink/renderer/core/exported/web_view_impl.cc

@@ -303,11 +303,13 @@ void WebViewImpl::CloseWindowSoon() {
 }
 
 void WebViewImpl::DoDeferredCloseWindowSoon() {
-  // The main widget is currently not active. The active main frame widget is
-  // in a different process.  Have the browser route the close request to the
-  // active widget instead, so that the correct unload handlers are run.
-  DCHECK(remote_main_frame_host_remote_);
-  remote_main_frame_host_remote_->RouteCloseEvent();
+  // The main widget is currently not active. The active main frame widget is in
+  // a different process. Have the browser route the close request to the active
+  // widget instead, so that the correct unload handlers are run. We do an early
+  // return instead of a DCHECK to guard against DidDetachRemoteMainFrame()
+  // being called between this method is schedule and when it's actually run.
+  if (remote_main_frame_host_remote_.is_bound())
+    remote_main_frame_host_remote_->RouteCloseEvent();
 }
 
 WebViewImpl::WebViewImpl(