[comments] Add a comment explaining JSArrayBuffer::is_external.

Change-Id: I048b7d5229686cf64bed6ec31d118ae1928028a5
Reviewed-on: https://chromium-review.googlesource.com/461939Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44199}

src/objects.h

@@ -10441,6 +10441,9 @@ class JSArrayBuffer: public JSObject {
   inline uint32_t bit_field() const;
   inline void set_bit_field(uint32_t bits);
 
+  // [is_external]: true indicates that the embedder is in charge of freeing the
+  // backing_store, while is_external == false means that v8 will free the
+  // memory block once all ArrayBuffers referencing it are collected by the GC.
   inline bool is_external();
   inline void set_is_external(bool value);