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This patch creates new URLLoaderResource objects for each request
since those objects can't be reused.

Calling Open() on an instance of URLLoaderResource requires that mode_
is set to MODE_WAITING_TO_OPEN. After Open() has been called mode_
will never reach that state again.

BUG=None

Review URL: https://codereview.chromium.org/582623002

Cr-Commit-Position: refs/heads/master@{#296155}