It's fine to use JS_OBJECT_TYPE for JSIteratorResult and only have a
preallocated initial map for them to avoid unnecessary polymorphism
from generators / builtin iterators. The instance type doesn't
provide any advantage, since we always have to treat JSIteratorResult
objects as regular JSObjects later.

R=yangguo@chromium.org
TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1680513002

Cr-Commit-Position: refs/heads/master@{#33800}