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

extensions/common/constants.h

Index: extensions/common/constants.h
diff --git a/extensions/common/constants.h b/extensions/common/constants.h
index 06f7f6f76e9d2cbdaa08b0e1c51151f242a8d89d..48f6d93f10891e2fcb8e4c803fc9cc43eac4415e 100644
--- a/extensions/common/constants.h
+++ b/extensions/common/constants.h
@@ -113,6 +113,11 @@ extern const char kWebStoreAppId[];
 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