Support structured clone atop v8::ValueSerializer behind a RuntimeEnabledFeature.

third_party/WebKit/Source/bindings/core/v8/ScriptValueSerializer.h

Index: third_party/WebKit/Source/bindings/core/v8/ScriptValueSerializer.h
diff --git a/third_party/WebKit/Source/bindings/core/v8/ScriptValueSerializer.h b/third_party/WebKit/Source/bindings/core/v8/ScriptValueSerializer.h
index 8b1d67030b2f4a879d4e0117f85fe862693874f2..55bfef6630fc5ef1306b6a30e55f60a16ee5292b 100644
--- a/third_party/WebKit/Source/bindings/core/v8/ScriptValueSerializer.h
+++ b/third_party/WebKit/Source/bindings/core/v8/ScriptValueSerializer.h
@@ -483,10 +483,14 @@ public:
     bool readVersion(uint32_t& version);
     void setVersion(uint32_t);
 
+    // Used to extract the underlying buffer, in order to bypass
+    // SerializedScriptValueReader.
+    const uint8_t* buffer() const { return m_buffer; }
+    unsigned length() const { return m_length; }
+
 protected:
     v8::Isolate* isolate() const { return m_scriptState->isolate(); }
     v8::Local<v8::Context> context() const { return m_scriptState->context(); }
-    unsigned length() const { return m_length; }
     unsigned position() const { return m_position; }
 
     const uint8_t* allocate(uint32_t size)