Use counter for legacy const.

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index 8ed20ee2121cfec8d313b042ac5ea878e66be628..7ab9be2fefed678e4234af649591bc8dffcfc3be 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -2272,6 +2272,7 @@ Block* Parser::ParseVariableDeclarations(
     if (is_sloppy(language_mode())) {
       mode = CONST_LEGACY;
       init_op = Token::INIT_CONST_LEGACY;
+      ++use_counts_[v8::Isolate::kLegacyConst];

[adamk, 2015/03/31 18:19:32]

Counting each const in a script separately seems a bit odd, especially since
Blink will collapse all these into one. But I guess that's not particularly
problematic. Just wondering if it's intentional that the V8 API for UseCounters
doesn't match how they're used in Blink.

     } else {
       DCHECK(var_context != kStatement);
       mode = CONST;