chrome/blink: Add functionality for in-product help for media elements.

third_party/WebKit/public/platform/media_download_in_product_help.mojom

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module blink.mojom;
+
+import "ui/gfx/geometry/mojo/geometry.mojom";
+
+// Browser-side service used by the renderer for showing the InProductHelp UI
+// widget/popup for the media download button.
+interface MediaDownloadInProductHelp {
+  // The position of the button in the frame's local coordinate space, where the
+  // UI widget should be anchored.
+  // The request_id denotes the element making the request.
+  ShowInProductHelpWidget(gfx.mojom.Rect button_rect);
+};