deps: V8: cherry-pick e3d7f8a

Original commit message:

    [build] update gen-postmortem-metadata for Python 3

    This change makes the code compatible with both Python 2 and Python 3.

    Change-Id: I99d68af9c3163607c3a2fdbafac339a98b7471e4
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1751331
    Commit-Queue: Yang Guo <yangguo@chromium.org>
    Reviewed-by: Yang Guo <yangguo@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#63207}

Refs: v8/v8@e3d7f8a

PR-URL: nodejs#29105
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

common.gypi

@@ -39,7 +39,7 @@
 
     # Reset this number to 0 on major V8 upgrades.
     # Increment by one for each non-official patch applied to deps/v8.
-    'v8_embedder_string': '-node.14',
+    'v8_embedder_string': '-node.15',
 
     ##### V8 defaults for Node.js #####
 

deps/v8/tools/gen-postmortem-metadata.py

@@ -640,9 +640,7 @@ def emit_config():
 
         out.write('/* class type information */\n');
         consts = [];
-        keys = typeclasses.keys();
-        keys.sort();
-        for typename in keys:
+        for typename in sorted(typeclasses):
                 klass = typeclasses[typename];
                 consts.append({
                     'name': 'type_%s__%s' % (klass, typename),
@@ -653,9 +651,7 @@ def emit_config():
 
         out.write('/* class hierarchy information */\n');
         consts = [];
-        keys = klasses.keys();
-        keys.sort();
-        for klassname in keys:
+        for klassname in sorted(klasses):
                 pklass = klasses[klassname]['parent'];
                 bklass = get_base_class(klassname);
                 if (bklass != 'Object'):