Add GetWebContents to ShellDelegate.

ash/shell/shell_delegate_impl.cc

Index: ash/shell/shell_delegate_impl.cc
diff --git a/ash/shell/shell_delegate_impl.cc b/ash/shell/shell_delegate_impl.cc
index 5da0db9e94e43c6fb0447a10c4dc1524af3f14a7..6cb197db5142e816eff4572d52b5b8fcd869ed20 100644
--- a/ash/shell/shell_delegate_impl.cc
+++ b/ash/shell/shell_delegate_impl.cc
@@ -175,5 +175,10 @@ void ShellDelegateImpl::SetTouchscreenEnabledInPrefs(bool enabled,
 
 void ShellDelegateImpl::UpdateTouchscreenStatusFromPrefs() {}
 
+content::WebContents* ShellDelegateImpl::GetWebContents() {
+  NOTIMPLEMENTED();
+  return nullptr;
+}
+
 }  // namespace shell
 }  // namespace ash