ServiceWorker: Add chrome://flags#enable-service-worker-script-streaming

This patch adds a flag to enable script streaming for service workers.
Design doc:
https://docs.google.com/document/d/1BbETxB2K1GeGUv4XIvGGforAJRRgYSNX5x4vPTGsCPU/edit

Bug: 683037
Change-Id: Iad104eb5497b4c2a904acae4a73e48545b39553e
Reviewed-on: https://chromium-review.googlesource.com/593707
Commit-Queue: Makoto Shimazu <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Cr-Commit-Position: refs/heads/master@{#492564}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 8085303d..5370c7f 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -651,6 +651,9 @@
 extern const char kServiceWorkerNavigationPreloadName[];
 extern const char kServiceWorkerNavigationPreloadDescription[];
 
+extern const char kServiceWorkerScriptStreamingName[];
+extern const char kServiceWorkerScriptStreamingDescription[];
+
 extern const char kSettingsWindowName[];
 extern const char kSettingsWindowDescription[];