Destroy WebWidgetTestClient when its WebWidget is destroyed.

content/shell/renderer/layout_test/layout_test_render_frame_observer.cc

Index: content/shell/renderer/layout_test/layout_test_render_frame_observer.cc
diff --git a/content/shell/renderer/layout_test/layout_test_render_frame_observer.cc b/content/shell/renderer/layout_test/layout_test_render_frame_observer.cc
index e86a22611c5331424ff9d05bde1b06310984fd66..240253cc57373c3508df89979305b80d29cbe0db 100644
--- a/content/shell/renderer/layout_test/layout_test_render_frame_observer.cc
+++ b/content/shell/renderer/layout_test/layout_test_render_frame_observer.cc
@@ -7,8 +7,10 @@
 #include <string>
 
 #include "components/test_runner/web_test_interfaces.h"
+#include "components/test_runner/web_test_proxy.h"
 #include "components/test_runner/web_test_runner.h"
 #include "content/public/renderer/render_frame.h"
+#include "content/public/test/layouttest_support.h"
 #include "content/shell/common/shell_messages.h"
 #include "content/shell/renderer/layout_test/blink_test_runner.h"
 #include "content/shell/renderer/layout_test/layout_test_render_thread_observer.h"
@@ -44,6 +46,15 @@ bool LayoutTestRenderFrameObserver::OnMessageReceived(
   return handled;
 }
 
+void LayoutTestRenderFrameObserver::FrameDetached() {
+  if (render_frame()->IsMainFrame()) {
+    test_runner::WebTestProxyBase* proxy =
+        GetWebTestProxyBase(render_frame()->GetRenderView());
+    proxy->set_web_widget(nullptr);
+    proxy->set_widget_test_client(nullptr);
+  }
+}
+
 void LayoutTestRenderFrameObserver::OnLayoutDumpRequest() {
   std::string dump =
       LayoutTestRenderThreadObserver::GetInstance()