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}
(cherry picked from commit 0cd7a9f853e3cb7c035b4ab9e07a503552267f9d)

Review URL: https://codereview.chromium.org/1927063002 .

Cr-Commit-Position: refs/branch-heads/2704@{#283}
Cr-Branched-From: 6e53600def8f60d8c632fadc70d7c1939ccea347-refs/heads/master@{#386251}
2 files changed