Use array<uint8> rather than string to pass BroadcastChannel messages.

third_party/WebKit/Source/modules/broadcastchannel/BroadcastChannel.cpp

Index: third_party/WebKit/Source/modules/broadcastchannel/BroadcastChannel.cpp
diff --git a/third_party/WebKit/Source/modules/broadcastchannel/BroadcastChannel.cpp b/third_party/WebKit/Source/modules/broadcastchannel/BroadcastChannel.cpp
index 676b82350874c1fa2f0589241ac0d461aeeba4c6..28fffa8ba2037b1fb698a0c0bf9ed95c6c4b7185 100644
--- a/third_party/WebKit/Source/modules/broadcastchannel/BroadcastChannel.cpp
+++ b/third_party/WebKit/Source/modules/broadcastchannel/BroadcastChannel.cpp
@@ -63,8 +63,11 @@ void BroadcastChannel::postMessage(const ScriptValue& message, ExceptionState& e
     if (exceptionState.hadException())
         return;
 
-    String data = value->toWireString();
-    m_remoteClient->OnMessage(data);
+    Vector<char> data;

[esprehn, 2016/07/23 08:53:50]

you want to call reserveInitialCapacity on both of these vectors to avoid
resizes and copies. :)

[Marijn Kruisselbrink, 2016/07/23 19:14:18]

Not sure what resized and copies that would avoid?
SerializedScriptValue::toWireBytes resizes the output vector appropriately
before adding anything to it, and similarly Vector::appendVector resizes before
appending all the data. So both these vectors will only get resized once anyway.
Maybe it would make sense to add a
SerializedScriptValue::toWireBytes(Vector<uint8_t>&) overload to avoid having to
go through the extra Vector<char>, as that's a copy that really shouldn't be
necessary.

+    value->toWireBytes(data);
+    Vector<uint8_t> mojoData;
+    mojoData.appendVector(data);

[dcheng, 2016/07/22 02:33:03]

It's sad that this results in copying the data now, especially since we're just
going from char to uint8_t...

[Marijn Kruisselbrink, 2016/07/22 03:23:12]

Yeah... but reinterpret_casting a Vector didn't seem like a great idea either. I
don't think this copy is really going to matter too much for the overall
performance anyway.

+    m_remoteClient->OnMessage(std::move(mojoData));
 }
 
 void BroadcastChannel::close()
@@ -95,10 +98,10 @@ DEFINE_TRACE(BroadcastChannel)
     EventTargetWithInlineData::trace(visitor);
 }
 
-void BroadcastChannel::OnMessage(const String& message)
+void BroadcastChannel::OnMessage(mojo::WTFArray<uint8_t> message)
 {
     // Queue a task to dispatch the event.
-    RefPtr<SerializedScriptValue> value = SerializedScriptValue::create(message);
+    RefPtr<SerializedScriptValue> value = SerializedScriptValue::create(reinterpret_cast<const char*>(&message.front()), message.size());
     MessageEvent* event = MessageEvent::create(nullptr, value.release(), getExecutionContext()->getSecurityOrigin()->toString());
     event->setTarget(this);
     bool success = getExecutionContext()->getEventQueue()->enqueueEvent(event);