[api] Add `Isolate::GetArrayBufferAllocator()`

This allows non-monolithic embedders to always allocate memory
for ArrayBuffer instances using the right allocation method.

This is based on a patch that Electron is currently using.

Refs: https://github.com/electron/electron/blob/1898f9162073910c05958295c612deec6121a892/patches/common/v8/array_buffer.patch
Change-Id: I39a614343118a0594aab48699a99cc2aad5b7ba9
Reviewed-on: https://chromium-review.googlesource.com/c/1462003Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59697}

include/v8.h

@@ -7818,6 +7818,9 @@ class V8_EXPORT Isolate {
    */
   void SetIdle(bool is_idle);
 
+  /** Returns the ArrayBuffer::Allocator used in this isolate. */
+  ArrayBuffer::Allocator* GetArrayBufferAllocator();
+
   /** Returns true if this isolate has a current context. */
   bool InContext();
 

src/api.cc

@@ -7979,6 +7979,11 @@ void Isolate::SetIdle(bool is_idle) {
   isolate->SetIdle(is_idle);
 }
 
+ArrayBuffer::Allocator* Isolate::GetArrayBufferAllocator() {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  return isolate->array_buffer_allocator();
+}
+
 bool Isolate::InContext() {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
   return !isolate->context().is_null();

test/cctest/test-api.cc

@@ -19999,6 +19999,7 @@ TEST(IsolateNewDispose) {
   CHECK_NOT_NULL(isolate);
   CHECK(current_isolate != isolate);
   CHECK(current_isolate == CcTest::isolate());
+  CHECK(isolate->GetArrayBufferAllocator() == CcTest::array_buffer_allocator());
 
   isolate->SetFatalErrorHandler(StoringErrorCallback);
   last_location = last_message = nullptr;