Remove ununsed WebFrameWidgetImpl DEFINE_TYPE_CAST

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

BUG=891908

Change-Id: I0156c1ba2fad75f50678d6446b53b1bf1b6b9ae9
Reviewed-on: https://chromium-review.googlesource.com/c/1489953
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635879}

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

@@ -208,12 +208,6 @@ class WebFrameWidgetImpl final : public WebFrameWidgetBase,
   SelfKeepAlive<WebFrameWidgetImpl> self_keep_alive_;
 };
 
-DEFINE_TYPE_CASTS(WebFrameWidgetImpl,
-                  WebFrameWidgetBase,
-                  widget,
-                  widget->ForSubframe(),
-                  widget.ForSubframe());
-
 }  // namespace blink
 
 #endif