[turbofan] Re-enable test that no longer fails.

This enables tests which rely on the context available at "debugger"
statements to be accurate. This is the case by now when deoptimization
information is available.

R=yangguo@chromium.org
BUG=v8:4055

Review URL: https://codereview.chromium.org/2129123002 .

Cr-Commit-Position: refs/heads/master@{#37604}

test/mjsunit/mjsunit.status

@@ -102,8 +102,6 @@
   'debug-stepout-scope-part2': [PASS, NO_VARIANTS],
   'debug-stepout-scope-part3': [PASS, NO_VARIANTS],
   'es6/debug-evaluate-blockscopes': [PASS, NO_VARIANTS],
-  # Issue 4055: Scope chain length observed by debugger is off.
-  'es6/generators-debug-scopes': [PASS, NO_VARIANTS],
 
   # TODO(titzer): --always-opt incorrectly disables CrankShaft soft deopt points
   'result-table-min': [PASS, NO_VARIANTS],