[arraybuffer] Allow nullptr backing store

I removed all uses of ArrayBufferContents that assumed that the start
of backing store is always not nullptr. Now we can also remove this
invariant altogether.

R=haraken@chromium.org

Bug: chromium:1008840
Change-Id: I479e502a986b4eb9ee3c6d56c7a99b6c677007bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904067
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717523}

third_party/blink/renderer/core/typed_arrays/array_buffer/array_buffer.cc

@@ -80,7 +80,7 @@ bool ArrayBuffer::ShareContentsWith(ArrayBufferContents& result) {
   DCHECK(IsShared());
   scoped_refptr<ArrayBuffer> keep_alive(this);
 
-  if (!contents_.DataShared()) {
+  if (!contents_.BackingStore()) {
     result.Detach();
     return false;
   }
@@ -93,7 +93,7 @@ bool ArrayBuffer::ShareNonSharedForInternalUse(ArrayBufferContents& result) {
   DCHECK(!IsShared());
   scoped_refptr<ArrayBuffer> keep_alive(this);
 
-  if (!contents_.Data()) {
+  if (!contents_.BackingStore()) {
     result.Detach();
     return false;
   }

third_party/blink/renderer/core/typed_arrays/array_buffer/array_buffer_contents.cc

@@ -71,30 +71,6 @@ ArrayBufferContents::ArrayBufferContents(
   }
 }
 
-ArrayBufferContents::ArrayBufferContents(
-    std::shared_ptr<v8::BackingStore> backing_store) {
-  if (!backing_store || backing_store->Data()) {
-    backing_store_ = std::move(backing_store);
-    return;
-  }
-  // ArrayBufferContents has to guarantee that Data() provides a valid pointer,
-  // even when DataSize() is '0'. That's why we create a new BackingStore here.
-
-  // TODO(ahaas): Remove this code here once nullptr is a valid result for
-  // Data().
-  CHECK_EQ(backing_store->ByteLength(), 0u);
-  void* data = AllocateMemoryOrNull(0, kDontInitialize);
-  CHECK_NE(data, nullptr);
-  DataDeleter deleter = [](void* data, size_t, void*) { FreeMemory(data); };
-  if (!backing_store->IsShared()) {
-    backing_store_ =
-        v8::ArrayBuffer::NewBackingStore(data, 0, deleter, nullptr);
-  } else {
-    backing_store_ =
-        v8::SharedArrayBuffer::NewBackingStore(data, 0, deleter, nullptr);
-  }
-}
-
 ArrayBufferContents::~ArrayBufferContents() = default;
 
 void ArrayBufferContents::Detach() {

third_party/blink/renderer/core/typed_arrays/array_buffer/array_buffer_contents.h

@@ -65,7 +65,8 @@ class CORE_EXPORT ArrayBufferContents {
                       InitializationPolicy);
   ArrayBufferContents(void* data, size_t length, DataDeleter deleter);
   ArrayBufferContents(ArrayBufferContents&&) = default;
-  explicit ArrayBufferContents(std::shared_ptr<v8::BackingStore> backing_store);
+  explicit ArrayBufferContents(std::shared_ptr<v8::BackingStore> backing_store)
+      : backing_store_(std::move(backing_store)) {}
 
   ~ArrayBufferContents();
 

third_party/blink/renderer/core/typed_arrays/dom_array_buffer.cc

@@ -80,6 +80,7 @@ v8::Local<v8::Object> DOMArrayBuffer::Wrap(
     v8::Context::Scope context_scope(creation_context->CreationContext());
     wrapper =
         v8::ArrayBuffer::New(isolate, Buffer()->Content()->BackingStore());
+
     wrapper->Externalize(Buffer()->Content()->BackingStore());
   }