[WeakPtr] Always retain a Flag in WeakReferenceOwner.

Rather than nulling the WeakReferenceOwner's |flag_| field when it is
invalidated, recreate the Flag so that there is always a valid one.

This is a precursor to requiring manual re-creation of the Flag in order
to create new WeakPtrs after having called InvalidateWeakPtrs().

Bug: 927987
Change-Id: Idbaa89c288a1b61cd22d4a7039aa60a5ad916a24
Reviewed-on: https://chromium-review.googlesource.com/c/1460491
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632060}

base/memory/weak_ptr.cc

@@ -34,6 +34,10 @@ bool WeakReference::Flag::MaybeValid() const {
   return !invalidated_.IsSet();
 }
 
+void WeakReference::Flag::DetachFromSequence() {
+  DETACH_FROM_SEQUENCE(sequence_checker_);
+}
+
 WeakReference::Flag::~Flag() = default;
 
 WeakReference::WeakReference() = default;
@@ -54,25 +58,24 @@ bool WeakReference::MaybeValid() const {
   return flag_ && flag_->MaybeValid();
 }
 
-WeakReferenceOwner::WeakReferenceOwner() = default;
+WeakReferenceOwner::WeakReferenceOwner()
+    : flag_(MakeRefCounted<WeakReference::Flag>()) {}
 
 WeakReferenceOwner::~WeakReferenceOwner() {
-  Invalidate();
+  flag_->Invalidate();
 }
 
 WeakReference WeakReferenceOwner::GetRef() const {
-  // If we hold the last reference to the Flag then create a new one.
+  // If we hold the last reference to the Flag then detach the SequenceChecker.
   if (!HasRefs())
-    flag_ = new WeakReference::Flag();
+    flag_->DetachFromSequence();
 
   return WeakReference(flag_);
 }
 
 void WeakReferenceOwner::Invalidate() {
-  if (flag_) {
   flag_->Invalidate();
-    flag_ = nullptr;
-  }
+  flag_ = MakeRefCounted<WeakReference::Flag>();
 }
 
 WeakPtrBase::WeakPtrBase() : ptr_(0) {}

base/memory/weak_ptr.h

@@ -102,6 +102,8 @@ class BASE_EXPORT WeakReference {
 
     bool MaybeValid() const;
 
+    void DetachFromSequence();
+
    private:
     friend class base::RefCountedThreadSafe<Flag>;
 
@@ -134,7 +136,7 @@ class BASE_EXPORT WeakReferenceOwner {
 
   WeakReference GetRef() const;
 
-  bool HasRefs() const { return flag_ && !flag_->HasOneRef(); }
+  bool HasRefs() const { return !flag_->HasOneRef(); }
 
   void Invalidate();
 
@@ -223,7 +225,6 @@ template <typename T>
 class WeakPtr : public internal::WeakPtrBase {
  public:
   WeakPtr() = default;
-
   WeakPtr(std::nullptr_t) {}
 
   // Allow conversion from U to T provided U "is a" T. Note that this