In addition to Array.prototype.values() which is already exposed, Blink
needs access to entries(), forEach() and keys() to properly set the
corresponding functions in value iterators for WebIDL conformance.

Add a few new entries to NATIVE_CONTEXT_IMPORTED_FIELDS and expand
V8_INTRINSICS_LIST, as well as some API tests for all these new exposed
functions.

BUG=chromium:632935
R=caitp@igalia.com,jochen@chromium.org,verwaest@chromium.org,yukishiino@chromium.org

Review-Url: https://codereview.chromium.org/2670833008
Cr-Commit-Position: refs/heads/master@{#43017}