Let IPC::SyncMessageFilter take advantage of thread-safe Send.

content/renderer/gpu/queue_message_swap_promise_unittest.cc

Index: content/renderer/gpu/queue_message_swap_promise_unittest.cc
diff --git a/content/renderer/gpu/queue_message_swap_promise_unittest.cc b/content/renderer/gpu/queue_message_swap_promise_unittest.cc
index 1261cb2bae6bc915e9c027d847b90eb575ed068d..76294fbd5299ac31a51d74d46fa93727aa96ec4f 100644
--- a/content/renderer/gpu/queue_message_swap_promise_unittest.cc
+++ b/content/renderer/gpu/queue_message_swap_promise_unittest.cc
@@ -31,7 +31,7 @@ class TestRenderWidget : public RenderWidget {
 
 class TestSyncMessageFilter : public IPC::SyncMessageFilter {
  public:
-  TestSyncMessageFilter() : IPC::SyncMessageFilter(NULL) {}
+  TestSyncMessageFilter() : IPC::SyncMessageFilter(NULL, false) {}

[jam, 2015/08/04 23:06:41]

it would be nice to remove this one usage, so that we can hide
SyncMessageFilter's constructor and have the only way to get it be from
SyncChannel. That way we can ensure it's not created incorrectly.

can we do this by making QueueMessageSwapPromise take an IPC::Sender and not a
SyncMessageFilter? i looked through the calls to GetRenderWidget::QueueMessage,
which from cs looks like what's leading to the vectors of messages being put in
it. i only see two calls to it, both with async IPCs. so I don't even know why
SyncMessageFilter was used. also i believe this is only called on the main
thread? in which case it doesn't make sense to use SyncMessageFilter.

would be good to check with the gpu folks who added that method

 
   bool Send(IPC::Message* message) override {
     messages_.push_back(message);