Make <webview> use out-of-process iframe architecture.

content/browser/browser_plugin/browser_plugin_guest.h

Index: content/browser/browser_plugin/browser_plugin_guest.h
diff --git a/content/browser/browser_plugin/browser_plugin_guest.h b/content/browser/browser_plugin/browser_plugin_guest.h
index 438fc1694e1f03694d4955aef3c73eab21e33f81..4cd91aba5fa960c7b13e53e348fe7ee4d8a43e65 100644
--- a/content/browser/browser_plugin/browser_plugin_guest.h
+++ b/content/browser/browser_plugin/browser_plugin_guest.h
@@ -79,6 +79,9 @@ struct DropData;
 // A BrowserPluginGuest can also create a new unattached guest via
 // CreateNewWindow. The newly created guest will live in the same partition,
 // which means it can share storage and can script this guest.
+//
+// Note: in --site-per-process, all IPCs sent out from this class will be
+// dropped on the floor since we don't have a BrowserPlugin.
 class CONTENT_EXPORT BrowserPluginGuest : public GuestHost,
                                           public WebContentsObserver {
  public: