Reland "Replace WTF::Function with base::OnceCallback."

This reverts commit 844e4a30.

Reason for revert:
The bug that caused a compilation error if DCHECKs is off is fixed.

Original change's description:
> Revert "Replace WTF::Function with base::OnceCallback."
>
> This reverts commit d298a4dd.
>
> Reason for revert:
>
> Findit (https://goo.gl/kROfz5) identified CL at revision 521649 as the
> culprit for failures in the build cycles as shown on:
> https://findit-for-me.appspot.com/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtL2QyOThhNGRkNzMyZjA5YzlhMWMzMDZhMjA2NTA3YzEyM2EzNzYxZTIM
>
> Sample Failed Build: https://ci.chromium.org/buildbot/chromium.chrome/Google%20Chrome%20Linux%20x64/24801
>
> Change-Id: I60fec5e188923122bb3b090acc49cfd0f0363df1
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 771087
> Reviewed-on: https://chromium-review.googlesource.com/808224
> Cr-Commit-Position: refs/heads/master@{#521653}

TBR=haraken@chromium.org,tzik@chromium.org

Bug: 771087
Change-Id: I0b74afe02f37d128acfe909fc5027f68afedf773
Reviewed-on: https://chromium-review.googlesource.com/808704
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Alexandr Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522377}

third_party/WebKit/Source/platform/WebTaskRunner.cpp

@@ -137,13 +137,6 @@ void WebTaskRunner::PostTask(const WebTraceLocation& location,
                   base::TimeDelta());
 }
 
-void WebTaskRunner::PostDelayedTask(const WebTraceLocation& location,
-                                    WTF::Closure task,
-                                    TimeDelta delay) {
-  DCHECK(RunsTasksInCurrentSequence());
-  PostDelayedTask(location, ConvertToBaseCallback(std::move(task)), delay);
-}
-
 TaskHandle WebTaskRunner::PostCancellableTask(const WebTraceLocation& location,
                                               WTF::Closure task) {
   DCHECK(RunsTasksInCurrentSequence());

third_party/WebKit/Source/platform/WebTaskRunner.h

@@ -78,7 +78,6 @@ class BLINK_PLATFORM_EXPORT WebTaskRunner
 
   // For same-thread posting. Must be called from the associated WebThread.
   void PostTask(const WebTraceLocation&, WTF::Closure);
-  void PostDelayedTask(const WebTraceLocation&, WTF::Closure, TimeDelta delay);
 
   // For same-thread cancellable task posting. Returns a TaskHandle object for
   // cancellation.

third_party/WebKit/Source/platform/wtf/Functional.h

@@ -327,44 +327,13 @@ namespace WTF {
 #endif
 
 template <typename Signature>
-class Function;
+using Function = base::OnceCallback<Signature>;
 
-template <typename R, typename... Args>
-class Function<R(Args...)> {
-  USING_FAST_MALLOC(Function);
-
- public:
-  Function() {}
-  Function(base::Callback<R(Args...)> callback)
-      : callback_(std::move(callback)) {}
-  ~Function() {}
-
-  Function(const Function&) = delete;
-  Function& operator=(const Function&) = delete;
-
-  Function(Function&& other) : callback_(std::move(other.callback_)) {}
-
-  Function& operator=(Function&& other) {
-    callback_ = std::move(other.callback_);
-    return *this;
-  }
-
-  R Run(Args... args) && {
-    return std::move(callback_).Run(std::forward<Args>(args)...);
-  }
-
-  bool IsCancelled() const { return callback_.IsCancelled(); }
-  void Reset() { callback_.Reset(); }
-  explicit operator bool() const { return static_cast<bool>(callback_); }
-
-  friend base::OnceCallback<R(Args...)> ConvertToBaseCallback(
-      Function function) {
-    return std::move(function.callback_);
-  }
-
- private:
-  base::Callback<R(Args...)> callback_;
-};
+template <typename Signature>
+base::OnceCallback<Signature> ConvertToBaseCallback(
+    Function<Signature> function) {
+  return function;
+}
 
 template <typename Signature>
 class CrossThreadFunction;
@@ -416,17 +385,17 @@ Function<base::MakeUnboundRunType<FunctionType, BoundParameters...>> Bind(
                     std::index_sequence_for<BoundParameters...>,
                     std::decay_t<BoundParameters>...>::ok,
                 "A bound argument uses a bad pattern.");
+  auto cb = base::BindOnce(function,
+                           std::forward<BoundParameters>(bound_parameters)...);
+#if DCHECK_IS_ON()
   using UnboundRunType =
       base::MakeUnboundRunType<FunctionType, BoundParameters...>;
-  auto cb =
-      base::Bind(function, std::forward<BoundParameters>(bound_parameters)...);
-#if DCHECK_IS_ON()
   using WrapperType =
-      ThreadCheckingCallbackWrapper<base::Callback<UnboundRunType>>;
-  cb = base::Bind(&WrapperType::Run,
-                  std::make_unique<WrapperType>(std::move(cb)));
+      ThreadCheckingCallbackWrapper<base::OnceCallback<UnboundRunType>>;
+  cb = base::BindOnce(&WrapperType::Run,
+                      std::make_unique<WrapperType>(std::move(cb)));
 #endif
-  return Function<UnboundRunType>(std::move(cb));
+  return cb;
 }
 
 template <typename FunctionType, typename... BoundParameters>
@@ -501,4 +470,6 @@ using WTF::Function;
 using WTF::CrossThreadFunction;
 using WTF::CrossThreadClosure;
 
+using WTF::ConvertToBaseCallback;
+
 #endif  // WTF_Functional_h