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.

tools/cef_parser.py

@@ -397,6 +397,7 @@ _simpletypes = {
     'char* const': ['char* const', 'NULL'],
     'cef_color_t': ['cef_color_t', '0'],
     'cef_json_parser_error_t': ['cef_json_parser_error_t', 'JSON_NO_ERROR'],
+    'cef_plugin_policy_t': ['cef_plugin_policy_t', 'PLUGIN_POLICY_ALLOW'],
     'CefCursorHandle' : ['cef_cursor_handle_t', 'kNullCursorHandle'],
     'CefEventHandle' : ['cef_event_handle_t', 'kNullEventHandle'],
     'CefWindowHandle' : ['cef_window_handle_t', 'kNullWindowHandle'],