Remove all but one use of WeakPtrFactory::DetachFromThread.

base/memory/weak_ptr.cc

Index: base/memory/weak_ptr.cc
diff --git a/base/memory/weak_ptr.cc b/base/memory/weak_ptr.cc
index 9dec8fd6db9c29956673a79326efa4b22af2f3ad..321b0576b2555ed995ac8a39c74bd40d944f155f 100644
--- a/base/memory/weak_ptr.cc
+++ b/base/memory/weak_ptr.cc
@@ -8,17 +8,23 @@ namespace base {
 namespace internal {
 
 WeakReference::Flag::Flag() : is_valid_(true) {
+  // Flags only become bound when checked for validity, or invalidated,
+  // so that we can check that later validity/invalidation operations on
+  // the same Flag take place on the same thread.
+  thread_checker_.DetachFromThread();
 }
 
 void WeakReference::Flag::Invalidate() {
   // The flag being invalidated with a single ref implies that there are no
   // weak pointers in existence. Allow deletion on other thread in this case.
-  DCHECK(thread_checker_.CalledOnValidThread() || HasOneRef());
+  DCHECK(thread_checker_.CalledOnValidThread() || HasOneRef())
+      << "WeakPtrs must be checked and invalidated on the same thread.";
   is_valid_ = false;
 }
 
 bool WeakReference::Flag::IsValid() const {
-  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(thread_checker_.CalledOnValidThread())
+      << "WeakPtrs must be checked and invalidated on the same thread.";
   return is_valid_;
 }
 
@@ -46,10 +52,10 @@ WeakReferenceOwner::~WeakReferenceOwner() {
 }
 
 WeakReference WeakReferenceOwner::GetRef() const {
-  // We also want to reattach to the current thread if all previous references
-  // have gone away.
+  // If we hold the last reference to the Flag then create a new one.
   if (!HasRefs())
     flag_ = new WeakReference::Flag();
+
   return WeakReference(flag_);
 }