Call OnFirstResponderChanged to simulate the View having focus.

Moving MouseLock emulation into the browser found that
the RenderWidgetHostViewMac::HasFocus always returned false
for WebTests. It is reasonable to simulate the view having
focus in WebTests so call OnFirstResponderChanged appropriately.

BUG=1097816

Change-Id: Id0a87cea174ac308bf73103a90fff678b30d429e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459568Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814913}

content/web_test/browser/web_test_shell_platform_delegate_mac.mm

@@ -164,17 +164,17 @@ void WebTestShellPlatformDelegate::ActivateContents(Shell* shell,
   for (Shell* window : Shell::windows()) {
     if (window != shell) {
       WebContents* other_top_contents = window->web_contents();
-      RenderWidgetHost* other_main_widget =
-          other_top_contents->GetMainFrame()->GetView()->GetRenderWidgetHost();
-      other_main_widget->Blur();
-      other_main_widget->SetActive(false);
+      auto* other_rwhv_mac = static_cast<RenderWidgetHostViewMac*>(
+          other_top_contents->GetMainFrame()->GetView());
+      other_rwhv_mac->OnFirstResponderChanged(false);
+      other_rwhv_mac->GetRenderWidgetHost()->SetActive(false);
     }
   }
 
-  RenderWidgetHost* main_widget =
-      top_contents->GetMainFrame()->GetView()->GetRenderWidgetHost();
-  main_widget->Focus();
-  main_widget->SetActive(true);
+  auto* top_rwhv_mac = static_cast<RenderWidgetHostViewMac*>(
+      top_contents->GetMainFrame()->GetView());
+  top_rwhv_mac->OnFirstResponderChanged(true);
+  top_rwhv_mac->GetRenderWidgetHost()->SetActive(true);
   activated_headless_shell_ = shell;
 }