Add ScopeInfo constants to post-mortem metadata

mdb_v8, a post-mortem debugging tool for Node.js, allows users to
inspect ScopeInfo structures in order to get more information about
closures.

Currently, it hardcodes the metadata it uses to find this information.
This change allows it to get this metadata from the node binary itself,
and thus to adapt to future changes made to the layout of the ScopeInfo
data structure.

BUG=

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1350843003

Cr-Commit-Position: refs/heads/master@{#30843}

src/objects.h

@@ -4054,7 +4054,6 @@ class ScopeInfo : public FixedArray {
   FOR_EACH_SCOPE_INFO_NUMERIC_FIELD(FIELD_ACCESSORS)
 #undef FIELD_ACCESSORS
 
- private:
   enum {
 #define DECL_INDEX(name) k##name,
     FOR_EACH_SCOPE_INFO_NUMERIC_FIELD(DECL_INDEX)
@@ -4062,6 +4061,7 @@ class ScopeInfo : public FixedArray {
     kVariablePartIndex
   };
 
+ private:
   // The layout of the variable part of a ScopeInfo is as follows:
   // 1. ParameterEntries:
   //    This part stores the names of the parameters for function scopes. One

tools/gen-postmortem-metadata.py

@@ -132,6 +132,15 @@ consts_misc = [
         'value': 'JavaScriptFrameConstants::kFunctionOffset' },
     { 'name': 'off_fp_args',
         'value': 'JavaScriptFrameConstants::kLastParameterOffset' },
+
+    { 'name': 'scopeinfo_idx_nparams',
+        'value': 'ScopeInfo::kParameterCount' },
+    { 'name': 'scopeinfo_idx_nstacklocals',
+        'value': 'ScopeInfo::kStackLocalCount' },
+    { 'name': 'scopeinfo_idx_ncontextlocals',
+        'value': 'ScopeInfo::kContextLocalCount' },
+    { 'name': 'scopeinfo_idx_first_vars',
+        'value': 'ScopeInfo::kVariablePartIndex' },
 ];
 
 #