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_dll/cpptoc/context_menu_handler_cpptoc.cc

@@ -15,6 +15,7 @@
 #include "libcef_dll/ctocpp/context_menu_params_ctocpp.h"
 #include "libcef_dll/ctocpp/frame_ctocpp.h"
 #include "libcef_dll/ctocpp/menu_model_ctocpp.h"
+#include "libcef_dll/ctocpp/run_context_menu_callback_ctocpp.h"
 
 
 namespace {
@@ -55,6 +56,49 @@ void CEF_CALLBACK context_menu_handler_on_before_context_menu(
       CefMenuModelCToCpp::Wrap(model));
 }
 
+int CEF_CALLBACK context_menu_handler_run_context_menu(
+    struct _cef_context_menu_handler_t* self, cef_browser_t* browser,
+    struct _cef_frame_t* frame, struct _cef_context_menu_params_t* params,
+    struct _cef_menu_model_t* model,
+    cef_run_context_menu_callback_t* callback) {
+  // AUTO-GENERATED CONTENT - DELETE THIS COMMENT BEFORE MODIFYING
+
+  DCHECK(self);
+  if (!self)
+    return 0;
+  // Verify param: browser; type: refptr_diff
+  DCHECK(browser);
+  if (!browser)
+    return 0;
+  // Verify param: frame; type: refptr_diff
+  DCHECK(frame);
+  if (!frame)
+    return 0;
+  // Verify param: params; type: refptr_diff
+  DCHECK(params);
+  if (!params)
+    return 0;
+  // Verify param: model; type: refptr_diff
+  DCHECK(model);
+  if (!model)
+    return 0;
+  // Verify param: callback; type: refptr_diff
+  DCHECK(callback);
+  if (!callback)
+    return 0;
+
+  // Execute
+  bool _retval = CefContextMenuHandlerCppToC::Get(self)->RunContextMenu(
+      CefBrowserCToCpp::Wrap(browser),
+      CefFrameCToCpp::Wrap(frame),
+      CefContextMenuParamsCToCpp::Wrap(params),
+      CefMenuModelCToCpp::Wrap(model),
+      CefRunContextMenuCallbackCToCpp::Wrap(callback));
+
+  // Return type: bool
+  return _retval;
+}
+
 int CEF_CALLBACK context_menu_handler_on_context_menu_command(
     struct _cef_context_menu_handler_t* self, cef_browser_t* browser,
     struct _cef_frame_t* frame, struct _cef_context_menu_params_t* params,
@@ -120,6 +164,7 @@ void CEF_CALLBACK context_menu_handler_on_context_menu_dismissed(
 CefContextMenuHandlerCppToC::CefContextMenuHandlerCppToC() {
   GetStruct()->on_before_context_menu =
       context_menu_handler_on_before_context_menu;
+  GetStruct()->run_context_menu = context_menu_handler_run_context_menu;
   GetStruct()->on_context_menu_command =
       context_menu_handler_on_context_menu_command;
   GetStruct()->on_context_menu_dismissed =