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/cef_messages.h

@@ -163,6 +163,11 @@ IPC_MESSAGE_ROUTED1(CefMsg_Response,
 IPC_MESSAGE_ROUTED1(CefMsg_ResponseAck,
                     int /* request_id */)
 
+// Tells the render frame to load all blocked plugins with the given identifier.
+// Based on ChromeViewMsg_LoadBlockedPlugins.
+IPC_MESSAGE_ROUTED1(CefViewMsg_LoadBlockedPlugins,
+                    std::string /* identifier */)
+
 // Sent on process startup to indicate whether this process is running in
 // incognito mode. Based on ChromeViewMsg_SetIsIncognitoProcess.
 IPC_MESSAGE_CONTROL1(CefProcessMsg_SetIsIncognitoProcess,