CachedMetadata support for ServiceWorker script.

Source/core/workers/WorkerGlobalScope.h

Index: Source/core/workers/WorkerGlobalScope.h
diff --git a/Source/core/workers/WorkerGlobalScope.h b/Source/core/workers/WorkerGlobalScope.h
index 0b037b6093192dd3569b522d45ab2d7772cc5770..a47152433186dd7cbefb08e07758043ce126735e 100644
--- a/Source/core/workers/WorkerGlobalScope.h
+++ b/Source/core/workers/WorkerGlobalScope.h
@@ -99,6 +99,8 @@ public:
 
     // WorkerUtils
     virtual void importScripts(const Vector<String>& urls, ExceptionState&);
+    virtual void setCachedMetadata(const KURL&, const char* data, size_t size) { }
+    virtual void clearCachedMetadata(const KURL&) { }

[kinuko, 2015/02/16 14:18:38]

Hmm.  We modify all the WorkerThreadStartupData, WorkerGlobalScope and
WorkerThread to add cached metadata just for ServiceWorker (for now), which
feels a bit intrusive to me.

I think... we could probably introduce more specialized classes instead? 
Something like:

- Define WorkerScriptCachedMetadataClient as a supplement of WorkerClients,
which has setCachedMetadata(), clearCachedMetadata() and releaseCachedMetadata()
methods
- Implement ServiceWorkerScriptCachedMetadataClient, which holds cached
metadata, and has reference to WebServiceWorkerContextClient
- Instantiate ServiceWorkerScriptCachedMetadataClient in WebEmbeddedWorkerImpl
and provide it to worker
- In WorkerThread.cpp we create MetadataHandler with MetadataClient which can be
obtained via worker clients supplement interface

[horo, 2015/02/16 16:20:12]

I want to use WorkerScriptCachedMetadataHandler in
WorkerGlobalScope::importScripts() to support v8 cache for imported scripts in
future.

In this case, I think WorkerScriptCachedMetadataClient as a supplement of
WorkerClients is not suitable.

[kinuko, 2015/02/17 02:32:31]

How do you plan to get the cached metadata for importScripts?

[horo, 2015/02/17 03:32:18]

I will create WorkerScriptCachedMetadataHandler using scriptLoader's
cachedMetadata in the for loop of WorkerGlobalScope::importScripts() and pass it
to m_script->evaluate().

I created a WIP patch https://codereview.chromium.org/932773002

[kinuko, 2015/02/17 05:19:24]

Thanks. So having the client hold cached metadata is not good, I see. I still
feel it'd be a bit cleaner to have a dedicated client interface for handling
setCachedMetadata() / clearCachedMetadata() methods, but that might be an
overkill. Hmm. (I'm going to chat with you)

     WorkerNavigator* navigator() const;
 
     // ExecutionContextClient