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/common/resource_bundle_impl.h

@@ -0,0 +1,31 @@
+// Copyright (c) 2015 The Chromium Embedded Framework Authors. All rights
+// reserved. Use of this source code is governed by a BSD-style license that can
+// be found in the LICENSE file.
+
+#ifndef CEF_LIBCEF_COMMON_RESOURCE_BUNDLE_IMPL_H_
+#define CEF_LIBCEF_COMMON_RESOURCE_BUNDLE_IMPL_H_
+#pragma once
+
+
+#include "include/cef_resource_bundle.h"
+
+class CefResourceBundleImpl : public CefResourceBundle {
+ public:
+  CefResourceBundleImpl();
+
+  // CefResourceBundle methods.
+  CefString GetLocalizedString(int string_id) override;
+  bool GetDataResource(int resource_id,
+                       void*& data,
+                       size_t& data_size) override;
+  bool GetDataResourceForScale(int resource_id,
+                               ScaleFactor scale_factor,
+                               void*& data,
+                               size_t& data_size) override;
+
+ private:
+  IMPLEMENT_REFCOUNTING(CefResourceBundleImpl);
+  DISALLOW_COPY_AND_ASSIGN(CefResourceBundleImpl);
+};
+
+#endif  // CEF_LIBCEF_COMMON_RESOURCE_BUNDLE_IMPL_H_