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There could be multiple URL requests at the same time. This patch
creates URL loader for each request.

Bug: b/153504737
Test: manual
Change-Id: I990f60e09242fc1ce65e23b8068aa0edda59601c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2141614Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763482}