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/ctocpp/request_handler_ctocpp.cc

@@ -17,7 +17,6 @@
 #include "libcef_dll/cpptoc/request_callback_cpptoc.h"
 #include "libcef_dll/cpptoc/response_cpptoc.h"
 #include "libcef_dll/cpptoc/sslinfo_cpptoc.h"
-#include "libcef_dll/cpptoc/web_plugin_info_cpptoc.h"
 #include "libcef_dll/ctocpp/request_handler_ctocpp.h"
 #include "libcef_dll/ctocpp/resource_handler_ctocpp.h"
 
@@ -375,36 +374,6 @@ bool CefRequestHandlerCToCpp::OnCertificateError(CefRefPtr<CefBrowser> browser,
   return _retval?true:false;
 }
 
-bool CefRequestHandlerCToCpp::OnBeforePluginLoad(CefRefPtr<CefBrowser> browser,
-    const CefString& url, const CefString& policy_url,
-    CefRefPtr<CefWebPluginInfo> info) {
-  cef_request_handler_t* _struct = GetStruct();
-  if (CEF_MEMBER_MISSING(_struct, on_before_plugin_load))
-    return false;
-
-  // AUTO-GENERATED CONTENT - DELETE THIS COMMENT BEFORE MODIFYING
-
-  // Verify param: browser; type: refptr_diff
-  DCHECK(browser.get());
-  if (!browser.get())
-    return false;
-  // Verify param: info; type: refptr_diff
-  DCHECK(info.get());
-  if (!info.get())
-    return false;
-  // Unverified params: url, policy_url
-
-  // Execute
-  int _retval = _struct->on_before_plugin_load(_struct,
-      CefBrowserCppToC::Wrap(browser),
-      url.GetStruct(),
-      policy_url.GetStruct(),
-      CefWebPluginInfoCppToC::Wrap(info));
-
-  // Return type: bool
-  return _retval?true:false;
-}
-
 void CefRequestHandlerCToCpp::OnPluginCrashed(CefRefPtr<CefBrowser> browser,
     const CefString& plugin_path) {
   cef_request_handler_t* _struct = GetStruct();