Adding flow traces for blink scheduler events

Source/platform/scheduler/TracedTask.cpp

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+#include "platform/scheduler/TracedTask.h"
+
+namespace blink {
+
+int TracedTask::s_nextFlowTraceID = 0;
+
+void TracedTask::run() const
+{
+    TRACE_EVENT_FLOW_END0("blink", m_traceName, TRACE_ID_MANGLE(m_flowTraceID));
+
+    TRACE_EVENT2("blink", m_traceName,
+        "src_file", m_location.fileName(),
+        "src_func", m_location.functionName());
+
+    m_task();
+}
+
+TracedTask::TracedTask(const Task& task, const TraceLocation& location, const char* traceName)
+    : m_task(task)
+    , m_location(location)
+    , m_traceName(traceName)
+{
+    bool tracingEnabled;
+    TRACE_EVENT_CATEGORY_GROUP_ENABLED("blink", &tracingEnabled);

[eseidel, 2014/09/04 06:49:13]

Is there also a check for flow events?  My understnding was that flow events
were a separate checkbox?

[Sami, 2014/09/04 09:51:07]

Only whole categories can be enabled or disabled. There isn't a way to select
individual event types within a category.

If this ends up being too noisy, we could move everything here under
"blink.scheduler".

+
+    if (tracingEnabled) {
+        // atomicIncrement is slow so we only do it if tracing is enabled
+        m_flowTraceID = atomicIncrement(&s_nextFlowTraceID);
+
+        TRACE_EVENT_FLOW_BEGIN2("blink", m_traceName, TRACE_ID_MANGLE(m_flowTraceID),
+        "src_file", m_location.fileName(),
+        "src_func", m_location.functionName());
+    }
+}
+
+} // namespace blink