Use correct creation context during Iterable.forEach iteration

Use |thisValue| instead of |scriptState->context()->Global()|, since this
is simpler and since Global() actually returns a WindowProxy object that
may change and become incorrect to use as creation context depending on
what the callback function does.

BUG=605910

Review URL: https://codereview.chromium.org/1918763002

Cr-Commit-Position: refs/heads/master@{#389785}