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/unittests/cookie_unittest.cc

@@ -900,6 +900,8 @@ class CookieTestJSHandler : public TestHandler {
   TrackCallback got_load_end2_;
   TrackCallback got_cookie1_;
   TrackCallback got_cookie2_;
+
+  IMPLEMENT_REFCOUNTING(CookieTestJSHandler);
 };
 
 }  // namespace
@@ -1186,6 +1188,8 @@ class CookieTestSchemeHandler : public TestHandler {
   TrackCallback got_cookie1_;
   TrackCallback got_cookie2_;
   TrackCallback got_cookie3_;
+
+  IMPLEMENT_REFCOUNTING(CookieTestSchemeHandler);
 };
 
 }  // namespace