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_creator.h

@@ -9,6 +9,7 @@
 #include "libcef/browser/menu_model_impl.h"
 
 #include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
 #include "content/public/browser/web_contents_observer.h"
 #include "content/public/common/context_menu_params.h"
 
@@ -17,6 +18,7 @@ class RenderFrameHost;
 class WebContents;
 };
 
+class CefRunContextMenuCallback;
 class CefBrowserHostImpl;
 
 class CefMenuCreator : public CefMenuModelImpl::Delegate,
@@ -58,11 +60,17 @@ class CefMenuCreator : public CefMenuModelImpl::Delegate,
   void MenuClosed(CefRefPtr<CefMenuModelImpl> source) override;
   bool FormatLabel(base::string16& label) override;
 
+  void ExecuteCommandCallback(int command_id,
+                              cef_event_flags_t event_flags);
+
   // Create the default menu model.
   void CreateDefaultModel();
   // Execute the default command handling.
   void ExecuteDefaultCommand(int command_id);
 
+  // Returns true if the specified id is a custom context menu command.
+  bool IsCustomContextMenuCommand(int command_id);
+
   // CefBrowserHostImpl pointer is guaranteed to outlive this object.
   CefBrowserHostImpl* browser_;
 
@@ -70,6 +78,12 @@ class CefMenuCreator : public CefMenuModelImpl::Delegate,
   content::ContextMenuParams params_;
   scoped_ptr<Runner> runner_;
 
+  // Not owned by this class.
+  CefRunContextMenuCallback* custom_menu_callback_;
+
+  // Must be the last member.
+  base::WeakPtrFactory<CefMenuCreator> weak_ptr_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(CefMenuCreator);
 };