[code-stubs] All keys in name dictionaries are internalized so stop checking in NameDictionaryLookupStub

Bug: v8:6921
Change-Id: Idaa4a7e31525ccd3f599115c54845aac2fcfa430
Reviewed-on: https://chromium-review.googlesource.com/707251
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48535}