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/libcef_dll.cc

@@ -58,6 +58,7 @@
 #include "libcef_dll/cpptoc/print_settings_cpptoc.h"
 #include "libcef_dll/cpptoc/process_message_cpptoc.h"
 #include "libcef_dll/cpptoc/request_callback_cpptoc.h"
+#include "libcef_dll/cpptoc/run_context_menu_callback_cpptoc.h"
 #include "libcef_dll/cpptoc/sslcert_principal_cpptoc.h"
 #include "libcef_dll/cpptoc/sslinfo_cpptoc.h"
 #include "libcef_dll/cpptoc/scheme_registrar_cpptoc.h"
@@ -248,6 +249,8 @@ CEF_EXPORT void cef_shutdown() {
   DCHECK(base::AtomicRefCountIsZero(
       &CefResourceBundleHandlerCToCpp::DebugObjCt));
   DCHECK(base::AtomicRefCountIsZero(&CefResourceHandlerCToCpp::DebugObjCt));
+  DCHECK(base::AtomicRefCountIsZero(
+      &CefRunContextMenuCallbackCppToC::DebugObjCt));
   DCHECK(base::AtomicRefCountIsZero(
       &CefRunFileDialogCallbackCToCpp::DebugObjCt));
   DCHECK(base::AtomicRefCountIsZero(&CefSSLCertPrincipalCppToC::DebugObjCt));