Add a counter for legacy const.

Only adds a counter id, does not count anything.

R=jochen@chromium.org
BUG=v8:3942
LOG=N

Review URL: https://codereview.chromium.org/1020883002

Cr-Commit-Position: refs/heads/master@{#27313}

include/v8.h

@@ -5070,6 +5070,7 @@ class V8_EXPORT Isolate {
   enum UseCounterFeature {
     kUseAsm = 0,
     kBreakIterator = 1,
+    kLegacyConst = 2,
     kUseCounterFeatureCount  // This enum value must be last.
   };