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.cc

@@ -10,6 +10,7 @@
 #include "base/bind.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
+#include "content/public/common/menu_item.h"
 #include "ui/base/accelerators/accelerator.h"
 
 namespace {
@@ -661,6 +662,41 @@ bool CefMenuModelImpl::VerifyRefCount() {
   return true;
 }
 
+void CefMenuModelImpl::AddMenuItem(const content::MenuItem& menu_item) {
+  const int command_id = static_cast<int>(menu_item.action);
+
+  switch (menu_item.type) {
+    case content::MenuItem::OPTION:
+      AddItem(command_id, menu_item.label);
+      break;
+    case content::MenuItem::CHECKABLE_OPTION:
+      AddCheckItem(command_id, menu_item.label);
+      break;
+    case content::MenuItem::GROUP:
+      AddRadioItem(command_id, menu_item.label, 0);
+      break;
+    case content::MenuItem::SEPARATOR:
+      AddSeparator();
+      break;
+    case content::MenuItem::SUBMENU: {
+      CefRefPtr<CefMenuModelImpl> sub_menu = static_cast<CefMenuModelImpl*>(
+          AddSubMenu(command_id, menu_item.label).get());
+      for (size_t i = 0; i < menu_item.submenu.size(); ++i)
+        sub_menu->AddMenuItem(menu_item.submenu[i]);
+      break;
+    }
+  }
+
+  if (!menu_item.enabled && menu_item.type != content::MenuItem::SEPARATOR)
+    SetEnabled(command_id, false);
+
+  if (menu_item.checked &&
+      (menu_item.type == content::MenuItem::CHECKABLE_OPTION ||
+       menu_item.type == content::MenuItem::GROUP)) {
+    SetChecked(command_id, true);
+  }
+}
+
 void CefMenuModelImpl::AppendItem(const Item& item) {
   ValidateItem(item);
   items_.push_back(item);