Introduce infrastructure for tracing ScriptWrappables.

third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.cpp

Index: third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.cpp
diff --git a/third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.cpp b/third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.cpp
index 3534a4ebfb5d2b1cd83854203e91f08945483b90..f93e985399aaf1c5f4538b0c320fe860df1fd053 100644
--- a/third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.cpp
+++ b/third_party/WebKit/Source/bindings/core/v8/ScriptWrappable.cpp
@@ -35,4 +35,9 @@ v8::Local<v8::Object> ScriptWrappable::associateWithWrapper(v8::Isolate* isolate
     return V8DOMWrapper::associateObjectWithWrapper(isolate, this, wrapperTypeInfo, wrapper);
 }
 
+void ScriptWrappable::markWrapperAlive(const v8::Persistent<v8::Object>& handle, v8::Isolate* isolate)

[haraken, 2016/04/13 12:26:22]

I think this static method should be moved to ScriptWrappableHeapTracer.
ScriptWrappableHeapTracer should be responsible for proxying things to V8 APIs.

[Marcel Hlopko, 2016/04/14 16:39:08]

Agreed. Done.

+{
+    handle.RegisterExternalReference(isolate);

[haraken, 2016/04/13 12:26:22]

This will keep alive the wrapper of the main world, but what about wrappers in
other isolated worlds?

[Marcel Hlopko, 2016/04/14 16:39:08]

For the first version, let's mark wrappers from all worlds when wrapper from any
world is reachable. I changed the code accordingly - ptal.
As for the future plans, we will have to pass context embedder data from V8, and
visitor would know for which world it traces. For the incremental
approach, we will have either 2 marking queues, one for the main world, one for
all other (having one queue per world seems "too much"), or one 
marking queue with pairs <scriptWrappable,world>. But that is distant future,
and we will discuss it more for sure.

[haraken, 2016/04/15 01:28:16]

This sounds like a good plan. For the first version, let's just mark wrappers in
all worlds, which will keep the existing behavior.

+}
+
 } // namespace blink