This removes brittle Scope walking in FinalizeForOfStatement()
by making ParserBase call InitializeForEachStatement() while
in the proper Scope.

Bug: v8:6724
Change-Id: I6e828ccb3a5e4d98633a95a2bfb8d255ad0fc0eb
Reviewed-on: https://chromium-review.googlesource.com/614654Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47376}