[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..dc9d5d2fac446c48565760455893958e952b1f48 100644
--- a/extensions/common/constants.h
+++ b/extensions/common/constants.h
@@ -113,6 +113,12 @@ 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 extension event listeners registered
+// from a non-service worker context
+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