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Previously URLSearchParamsIterationSource iterated over a copy of
URLSearchParams, but it is specified in the standard as iterating over
the live list.

Modify it to reference the original URLSearchParams object instead of
making a copy.

Also add web-platform tests for delete during foreach, and a chromium-
specific test for GC of URLSearchParams during iteration.

Remove failing expectations.

BUG=677322

Change-Id: I8c53fd8dd9863fe1146c5b7849d4f08245b37bc1
Reviewed-on: https://chromium-review.googlesource.com/987839Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548363}