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 86278b98694810af4cffc6ec4fa01b6f71f4346d..cbb99765c0b7b336a0dbb44712dac51e8d79c955 100644
--- a/content/browser/browser_plugin/browser_plugin_guest.h
+++ b/content/browser/browser_plugin/browser_plugin_guest.h
@@ -77,6 +77,9 @@ 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: