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Original method expects only a single equal sign in each
query parameter and that the query string is only in lower
case. However, body may contain a URL with may have an
unescaped equal sign. And query string keys may be in mixed
case.

As part of this fix, also removed -supportedHeaders from
public API and use GURL for query parameter processing.

Bug: 747949
Change-Id: Ia157983e946d7301300d22f41e007bd10ce8b786
Reviewed-on: https://chromium-review.googlesource.com/594873
Commit-Queue: Peter Lee <pkl@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491280}