[d8] Use ValueSerializer for postMessage (instead of ad-hoc serializer)

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 2f544cb1bb7297578abf14d8b53d0c42d9ec4cb8..0d521093b15e696ad7210fe7bf7fc998782ec46d 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -1740,6 +1740,10 @@ class V8_EXPORT ValueSerializer {
      * Allocates memory for the buffer of at least the size provided. The actual
      * size (which may be greater or equal) is written to |actual_size|. If no
      * buffer has been allocated yet, nullptr will be provided.
+     *
+     * If the memory cannot be allocated, nullptr should be returned.
+     * |actual_size| will be ignored. It is assumed that |old_buffer| is still
+     * valid in this case and has not been modified.
      */
     virtual void* ReallocateBufferMemory(void* old_buffer, size_t size,
                                          size_t* actual_size);