[extension SW]: Support event listener registration and event dispatching.

extensions/common/constants.h

 // Copyright (c) 2012 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.
 
 #ifndef EXTENSIONS_COMMON_CONSTANTS_H_
 #define EXTENSIONS_COMMON_CONSTANTS_H_
 
 #include "base/files/file_path.h"
 #include "ui/base/layout.h"
 
@@ skipping 95 matching lines @@
 // content/public/common.
 extern const int64_t kInvalidServiceWorkerVersionId;
 
 // The extension id of the Web Store component application.
 extern const char kWebStoreAppId[];
 
 // The key used for signing some pieces of data from the webstore.
 extern const uint8_t kWebstoreSignaturesPublicKey[];
 extern const size_t kWebstoreSignaturesPublicKeySize;
 
+// A thread identifier used in extension events where the thread id in question
+// does not belong to a worker thread.
+// This is the default thread id used for non service worker extension events.

[Devlin, 2017/06/01 04:54:50]

nit: s/non service worker extension events/extension event listeners registered
from a non-service worker context
or similar

[lazyboy, 2017/06/01 23:33:29]

Done.

+extern const int kNonWorkerThreadId;
+
 // Enumeration of possible app launch sources.
 // This should be kept in sync with LaunchSource in
 // extensions/common/api/app_runtime.idl, and GetLaunchSourceEnum() in
 // extensions/browser/api/app_runtime/app_runtime_api.cc.
 // Note the enumeration is used in UMA histogram so entries
 // should not be re-ordered or removed.
 enum AppLaunchSource {
   SOURCE_NONE,
   SOURCE_UNTRACKED,
   SOURCE_APP_LAUNCHER,
@@ skipping 93 matching lines @@
 
 // Extension ids used by Hangouts.
 extern const char* const kHangoutsExtensionIds[6];
 
 // Error message when enterprise policy blocks scripting of webpage.
 extern const char kPolicyBlockedScripting[];
 
 }  // namespace extension_misc
 
 #endif  // EXTENSIONS_COMMON_CONSTANTS_H_