[api] Reorder fields in {SharedArrayBuffer,ArrayBuffer}::Contents.

This is for backwards ABI compatibility to version 6.0.

R=bmeurer@chromium.org

Bug: v8:6592
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I20f3fa79324511c6e393ddaad50880f22ef6df77
Reviewed-on: https://chromium-review.googlesource.com/569758Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46625}

include/v8.h

@@ -4284,11 +4284,11 @@ class V8_EXPORT ArrayBuffer : public Object {
   class V8_EXPORT Contents { // NOLINT
    public:
     Contents()
-        : allocation_base_(nullptr),
+        : data_(nullptr),
+          byte_length_(0),
+          allocation_base_(nullptr),
           allocation_length_(0),
-          allocation_mode_(Allocator::AllocationMode::kNormal),
-          data_(nullptr),
-          byte_length_(0) {}
+          allocation_mode_(Allocator::AllocationMode::kNormal) {}
 
     void* AllocationBase() const { return allocation_base_; }
     size_t AllocationLength() const { return allocation_length_; }
@@ -4300,11 +4300,11 @@ class V8_EXPORT ArrayBuffer : public Object {
     size_t ByteLength() const { return byte_length_; }
 
    private:
+    void* data_;
+    size_t byte_length_;
     void* allocation_base_;
     size_t allocation_length_;
     Allocator::AllocationMode allocation_mode_;
-    void* data_;
-    size_t byte_length_;
 
     friend class ArrayBuffer;
   };
@@ -4654,11 +4654,11 @@ class V8_EXPORT SharedArrayBuffer : public Object {
   class V8_EXPORT Contents {  // NOLINT
    public:
     Contents()
-        : allocation_base_(nullptr),
+        : data_(nullptr),
+          byte_length_(0),
+          allocation_base_(nullptr),
           allocation_length_(0),
-          allocation_mode_(ArrayBuffer::Allocator::AllocationMode::kNormal),
-          data_(nullptr),
-          byte_length_(0) {}
+          allocation_mode_(ArrayBuffer::Allocator::AllocationMode::kNormal) {}
 
     void* AllocationBase() const { return allocation_base_; }
     size_t AllocationLength() const { return allocation_length_; }
@@ -4670,11 +4670,11 @@ class V8_EXPORT SharedArrayBuffer : public Object {
     size_t ByteLength() const { return byte_length_; }
 
    private:
+    void* data_;
+    size_t byte_length_;
     void* allocation_base_;
     size_t allocation_length_;
     ArrayBuffer::Allocator::AllocationMode allocation_mode_;
-    void* data_;
-    size_t byte_length_;
 
     friend class SharedArrayBuffer;
   };