Make latency tests measure until the end of swap buffers.

content/common/gpu/gpu_command_buffer_stub.cc

Index: content/common/gpu/gpu_command_buffer_stub.cc
diff --git a/content/common/gpu/gpu_command_buffer_stub.cc b/content/common/gpu/gpu_command_buffer_stub.cc
index ce31401a6358aa652b1b365f062954aa59e1b111..f1e3770018e1a6954a704048a0ccd4c61773b164 100644
--- a/content/common/gpu/gpu_command_buffer_stub.cc
+++ b/content/common/gpu/gpu_command_buffer_stub.cc
@@ -181,6 +181,8 @@ bool GpuCommandBufferStub::HasUnprocessedCommands() {
 }
 
 void GpuCommandBufferStub::OnEcho(const IPC::Message& message) {
+  // For latency_tests.cc:
+  UNSHIPPED_TRACE_EVENT_INSTANT0("test_gpu", "CompositorSwapBuffersComplete");
   TRACE_EVENT0("gpu", "GpuCommandBufferStub::OnEcho");
   Send(new IPC::Message(message));
 }