[snapshot] update comment wrt code cache layout

R=jgruber@chromium.org

Change-Id: I07d9c2f952f0ca3cf50f5faa4ad8db78be2a5b04
Reviewed-on: https://chromium-review.googlesource.com/c/1342926Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57642}

src/snapshot/code-serializer.h

@@ -102,16 +102,15 @@ class SerializedCodeData : public SerializedData {
 
   // The data header consists of uint32_t-sized entries:
   // [0] magic number and (internally provided) external reference count
-  // [1] extra (API-provided) external reference count
-  // [2] version hash
-  // [3] source hash
-  // [4] cpu features
-  // [5] flag hash
+  // [1] version hash
+  // [2] source hash
+  // [3] cpu features
+  // [4] flag hash
+  // [5] number of reservation size entries
   // [6] number of code stub keys
-  // [7] number of reservation size entries
-  // [8] payload length
-  // [9] payload checksum part A
-  // [10] payload checksum part B
+  // [7] payload length
+  // [8] payload checksum part A
+  // [9] payload checksum part B
   // ...  reservations
   // ...  code stub keys
   // ...  serialized payload