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.

tests/cefclient/browser/client_app_browser.h

@@ -21,7 +21,11 @@ class ClientAppBrowser : public ClientApp,
   // constructor. See CefBrowserProcessHandler for documentation.
   class Delegate : public virtual CefBase {
    public:
-    virtual void OnContextInitialized(CefRefPtr<ClientApp> app) {}
+    virtual void OnBeforeCommandLineProcessing(
+        CefRefPtr<ClientAppBrowser> app,
+        CefRefPtr<CefCommandLine> command_line) {}
+
+    virtual void OnContextInitialized(CefRefPtr<ClientAppBrowser> app) {}
 
     virtual void OnBeforeChildProcessLaunch(
         CefRefPtr<ClientAppBrowser> app,