Implement WTF::WeakPtr in terms of base::WeakPtr

third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp

Index: third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp
diff --git a/third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp b/third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp
index 4912d3c6267a4be3b30792e8914859ce784d9dd3..8e6680f0ea51c8715500e1668a165629e1eabea5 100644
--- a/third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp
+++ b/third_party/WebKit/Source/core/dom/MainThreadTaskRunner.cpp
@@ -40,6 +40,8 @@ MainThreadTaskRunner::MainThreadTaskRunner(ExecutionContext* context)
     , m_pendingTasksTimer(this, &MainThreadTaskRunner::pendingTasksTimerFired)
     , m_suspended(false)
     , m_weakFactory(this)
+    // Bind a WeakPtr now to avoid data races creating a WeakPtr inside postTask.
+    , m_weakPtr(m_weakFactory.createWeakPtr())
 {
 }
 
@@ -51,7 +53,7 @@ void MainThreadTaskRunner::postTaskInternal(const WebTraceLocation& location, st
 {
     Platform::current()->mainThread()->getWebTaskRunner()->postTask(location, crossThreadBind(
         &MainThreadTaskRunner::perform,
-        m_weakFactory.createWeakPtr(),
+        m_weakPtr,

[esprehn, 2016/09/13 19:44:21]

This means we're sharing a weakptr now instead of making a new one per task?

[dcheng, 2016/09/13 19:51:40]

It's copied, not shared, when it's bound into the task. From an overhead
perspective, we take a small hit, since we have to keep around a WeakPtr and a
WeakPtrFactory.

Once this lands though, I have a patch to reduce the overhead of WeakPtr a
little bit, and I will also explore the feasibility of making it safe to call
GetWeakPtr() on multiple threads if there are no explicit InvalidateWeakPtr()
calls.

         passed(std::move(task)),
         isInspectorTask,
         instrumenting));