GuestView: Cleanup on Embedder process swap

BUG=408688
Test=Launch chrome://extensions with --enable-embedded-extension-options, open SPDY options, navigate to www.google.com, the "Options: SPDY Proxy Extension Options" entry should disappear from the Task Manager.

Review URL: https://codereview.chromium.org/514653003

Cr-Commit-Position: refs/heads/master@{#292505}

extensions/browser/guest_view/guest_view_base.cc

@@ -70,6 +70,12 @@ class GuestViewBase::EmbedderWebContentsObserver : public WebContentsObserver {
     Destroy();
   }
 
+  virtual void RenderViewHostChanged(
+      content::RenderViewHost* old_host,
+      content::RenderViewHost* new_host) OVERRIDE {
+    Destroy();
+  }
+
   virtual void RenderProcessGone(base::TerminationStatus status) OVERRIDE {
     Destroy();
   }