Use a named constant for the object id size when writing out the header.

runtime/vm/heap_profiler.cc

Index: runtime/vm/heap_profiler.cc
diff --git a/runtime/vm/heap_profiler.cc b/runtime/vm/heap_profiler.cc
index 1d34b2d31276b2fdb65d226f4a0b7dce1c09859b..1c69562e5543a86beb3642887368e0b96746eeb3 100644
--- a/runtime/vm/heap_profiler.cc
+++ b/runtime/vm/heap_profiler.cc
@@ -305,7 +305,7 @@ void HeapProfiler::Write(const void* data, intptr_t size) {
 void HeapProfiler::WriteHeader() {
   const char magic[] = "JAVA PROFILE 1.0.1";
   Write(magic, sizeof(magic));
-  uint32_t size = htonl(8);
+  uint32_t size = htonl(kObjectIdSize);
   Write(&size, sizeof(size));
   uint64_t milliseconds = OS::GetCurrentTimeMillis();
   uint32_t hi = htonl((uint32_t)((milliseconds >> 32) & 0x00000000FFFFFFFF));