[snapshot] Add tracking for read-only snapshot size.

Bug: v8:8329
Change-Id: I5be972698809ca77a621bb960cbc6a23b9f0f4b0
Reviewed-on: https://chromium-review.googlesource.com/c/1296474Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56901}

test/memory/Memory.json

@@ -23,6 +23,10 @@
       "name": "SnapshotSizeBuiltins",
       "results_regexp": "(\\d+) bytes for builtins$"
     },
+    {
+      "name": "SnapshotSizeReadOnly",
+      "results_regexp": "(\\d+) bytes for read-only$"
+    },
     {
       "name": "SnapshotSizeContext",
       "results_regexp": "(\\d+) bytes in \\d+ chunks for context #0$"