Add plugin placeholder and policy support (issue #1708)

- Default plugin loading policy can be specified using the new
  `--plugin-policy=[allow|block|detect]` command-line flag.
- Move CefRequestHandler::OnBeforePluginLoad to
  CefRequestContextHandler and add a new policy argument that
  supports different actions (allow, block, detect, disable) on a
  per-plugin-instance basis.
- Add CefContextMenuHandler::RunContextMenu for providing a custom
  context menu implementation.
- Add CefResourceBundleHandler::GetDataResourceForScale for
  returning scaled resources (issue #1272).
- Add CefResourceBundle for retrieving resources from the resource
  bundle (*.pak) files loaded by CEF during startup or via the
  CefResourceBundleHandler.
- Linux: Fix Debug build IO access warning with CefGetMimeType.
- cef_unittests: Move the refcounting implementation from TestHandler
  to subclasses in order to support interface inheritance from
  subclasses.

libcef/browser/menu_model_impl.h

@@ -15,6 +15,10 @@
 #include "base/threading/platform_thread.h"
 #include "ui/base/models/menu_model.h"
 
+namespace content {
+struct MenuItem;
+}
+
 class CefMenuModelImpl : public CefMenuModel {
  public:
   class Delegate {
@@ -112,6 +116,9 @@ class CefMenuModelImpl : public CefMenuModel {
   // Verify that only a single reference exists to all CefMenuModelImpl objects.
   bool VerifyRefCount();
 
+  // Helper for adding custom menu items originating from the renderer process.
+  void AddMenuItem(const content::MenuItem& menu_item);
+
   ui::MenuModel* model() { return model_.get(); }
   Delegate* delegate() { return delegate_; }
   void set_delegate(Delegate* delegate) { delegate_ = NULL; }