Switch WebFrameWidgetBase to use downcast helpers

Apart from start making use of the always-true downcast
helper, in order to ensure that WebFrameWidget is always a
WebFrameWidgetBase intance (and hence respecting the downcast
helper contract), this CL also adds a private WebFrameWidget
ctor, and make it a friend-class to WebFrameWidgetBase.

This ensures that WebFrameWidgetBase is the only concrete subclass
that implements WebFrameWidget.

R=jbroman@chromium.org
CC=blink-reviews-vendor@chromium.org

BUG=891908

Change-Id: Ibdea5aea0fcb538b15a12a7cfc00691249ef2099
Reviewed-on: https://chromium-review.googlesource.com/c/1490291
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636234}

third_party/blink/public/web/web_frame_widget.h

@@ -132,6 +132,13 @@ class WebFrameWidget : public WebWidget {
   // This function provides zooming for find in page results when browsing with
   // page autosize.
   virtual void ZoomToFindInPageRect(const WebRect& rect_in_root_frame) = 0;
+
+ private:
+  // This private constructor and the class/friend declaration ensures that
+  // WebFrameWidgetBase is the only concrete subclass that implements
+  // WebFrameWidget, so that it is safe to downcast to WebFrameWidgetBase.
+  friend class WebFrameWidgetBase;
+  WebFrameWidget() = default;
 };
 
 }  // namespace blink

third_party/blink/renderer/core/exported/web_page_popup_impl.cc

@@ -214,7 +214,7 @@ class PagePopupChromeClient final : public EmptyChromeClient {
     WebFrameWidget* widget = web_frame->LocalRoot()->FrameWidget();
     if (!widget)
       return;
-    if (WebWidgetClient* client = ToWebFrameWidgetBase(widget)->Client())
+    if (WebWidgetClient* client = To<WebFrameWidgetBase>(widget)->Client())
       client->SetTouchAction(static_cast<WebTouchAction>(touch_action));
   }
 

third_party/blink/renderer/core/frame/web_frame_widget_base.h

@@ -16,6 +16,7 @@
 #include "third_party/blink/renderer/core/dom/user_gesture_indicator.h"
 #include "third_party/blink/renderer/platform/graphics/paint/paint_image.h"
 #include "third_party/blink/renderer/platform/heap/member.h"
+#include "third_party/blink/renderer/platform/wtf/casting.h"
 
 namespace cc {
 class AnimationHost;
@@ -200,7 +201,12 @@ class CORE_EXPORT WebFrameWidgetBase
   friend class WebViewImpl;
 };
 
-DEFINE_TYPE_CASTS(WebFrameWidgetBase, WebFrameWidget, widget, true, true);
+template <>
+struct DowncastTraits<WebFrameWidgetBase> {
+  // All concrete implementations of WebFrameWidget are derived from
+  // WebFrameWidgetBase.
+  static bool AllowFrom(const WebFrameWidget& widget) { return true; }
+};
 
 }  // namespace blink