<webview>: Partially migrate loadcommit event from content to chrome

chrome/browser/extensions/api/webview/webview_api_constants.cc

+// Copyright (c) 2013 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.
+
+#include "chrome/browser//extensions/api/webview/webview_api_constants.h"
+
+namespace webview_api_constants {
+
+// Events.
+const char kEventLoadCommit[] = "webview.onLoadCommit";
+
+// Parameters/properties on events.
+const char kIsTopLevel[] = "isTopLevel";
+const char kUrl[] = "url";
+
+}  // namespace webview_api_constants