Fix ArrayBuffer::Allocator::AllocationMode lookup

CL https://chromium-review.googlesource.com/c/523271/6 introduced a
wrong lookup of ArrayBuffer::Allocator::Allocator::AllocationMode that
breaks compilation with Visual Studio 2013.

R=jochen@chromium.org

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I160242bc24ef8c23574d3ec3734be822a681b408
Reviewed-on: https://chromium-review.googlesource.com/548399Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46240}

include/v8.h

@@ -4665,7 +4665,7 @@ class V8_EXPORT SharedArrayBuffer : public Object {
    private:
     void* allocation_base_;
     size_t allocation_length_;
-    ArrayBuffer::Allocator::Allocator::AllocationMode allocation_mode_;
+    ArrayBuffer::Allocator::AllocationMode allocation_mode_;
     void* data_;
     size_t byte_length_;