[snapshot] add tracking for builtin snapshot size.

R=jgruber@chromium.org

Bug: v8:6624
Change-Id: I4929a404999cf4d2c12471d5ee13533234fbcf7e
Reviewed-on: https://chromium-review.googlesource.com/645126Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47730}

test/memory/Memory.json

@@ -18,6 +18,10 @@
       "name": "SnapshotSizeStartup",
       "results_regexp": "(\\d+) bytes for startup$"
     },
+    {
+      "name": "SnapshotSizeBuiltins",
+      "results_regexp": "(\\d+) bytes for builtins$"
+    },
     {
       "name": "SnapshotSizeContext",
       "results_regexp": "(\\d+) bytes for context #0$"