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/browser_main.cc

@@ -24,6 +24,7 @@
 #include "base/command_line.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/string_number_conversions.h"
+#include "chrome/browser/plugins/plugin_finder.h"
 #include "content/browser/webui/content_web_ui_controller_factory.h"
 #include "content/public/browser/child_process_security_policy.h"
 #include "content/public/browser/gpu_data_manager.h"
@@ -165,6 +166,9 @@ void CefBrowserMainParts::PreMainMessageLoopRun() {
     }
   }
 
+  // Triggers initialization of the singleton instance on UI thread.
+  PluginFinder::GetInstance()->Init();
+
 #if defined(OS_WIN)
   PlatformPreMainMessageLoopRun();
 #endif