text input focus: RemoveObserver when RenderProcessHost exits.

Makes sure that a WebView as RenderProcessHostObserver is removed from RenderProcessHost's observer list when RenderProcessHost exits.

BUG=290701

Review URL: https://codereview.chromium.org/878383002

Cr-Commit-Position: refs/heads/master@{#313662}

ui/views/controls/webview/webview.cc

@@ -271,6 +271,7 @@ void WebView::RenderProcessExited(content::RenderProcessHost* host,
 
 void WebView::RenderProcessHostDestroyed(content::RenderProcessHost* host) {
   DCHECK_EQ(host, observing_render_process_host_);
+  observing_render_process_host_->RemoveObserver(this);
   observing_render_process_host_ = nullptr;
 }