Re-enable regress-crbug-501711 and regress-4279 for --isolates tests

BUG=v8:4378
TBR=yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/1305583002 .

Cr-Commit-Position: refs/heads/master@{#30258}

test/mjsunit/mjsunit.status

@@ -252,10 +252,6 @@
   # BUG(chromium:508074). Remove this once the issue is fixed.
   'harmony/arrow-rest-params': [PASS, NO_VARIANTS],
   'harmony/rest-params': [PASS, ['no_snap == True', NO_VARIANTS]],
-
-  # BUG(v8:4378).
-  'regress/regress-crbug-501711': [PASS, ['isolates', SKIP]],
-  'regress/regress-4279': [PASS, ['isolates', SKIP]],
 }],  # ALWAYS
 
 ['novfp3 == True', {