Remove blink::PersistentBase::AtomicGet.

Writes to cross-thread persistents are no longer atomic (release) stores, so
this is not more safe than a non-atomic access -- its correctness depends on
external synchronization, which at the sole call site is provided by
ProcessHeap::CrossThreadPersistentMutex(), which also guards all writes to
PersistentBase::raw_ for cross-thread handles.

Bug: 736037
Change-Id: Ic47ef65b731030cdf9897f2d7dd48899514eea5e
Reviewed-on: https://chromium-review.googlesource.com/c/1351567Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611486}

third_party/blink/renderer/platform/heap/persistent.h

@@ -197,13 +197,6 @@ class PersistentBase {
     region.FreePersistentNode(persistent_node_);
   }
 
- protected:
-  NO_SANITIZE_ADDRESS
-  T* AtomicGet() {
-    return reinterpret_cast<T*>(AcquireLoad(reinterpret_cast<void* volatile*>(
-        const_cast<typename std::remove_const<T>::type**>(&raw_))));
-  }
-
  private:
   NO_SANITIZE_ADDRESS
   void Assign(T* ptr) {
@@ -511,8 +504,6 @@ class CrossThreadPersistent
   CrossThreadPersistent(const Member<U>& other) : Parent(other) {}
   CrossThreadPersistent(WTF::HashTableDeletedValueType x) : Parent(x) {}
 
-  T* AtomicGet() { return Parent::AtomicGet(); }
-
   // Instead of using release(), assign then clear() instead.
   // Using release() with per thread heap enabled can cause the object to be
   // destroyed before assigning it to a new handle.

third_party/blink/renderer/platform/heap/persistent_node.cc

@@ -182,7 +182,7 @@ void CrossThreadPersistentRegion::PrepareForThreadStateTermination(
           reinterpret_cast<CrossThreadPersistent<DummyGCBase>*>(
               slots->slot_[i].Self());
       DCHECK(persistent);
-      void* raw_object = persistent->AtomicGet();
+      void* raw_object = persistent->Get();
       if (!raw_object)
         continue;
       BasePage* page = PageFromObject(raw_object);