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Define ARC FileProvider URL constans in arc/fileapi/arc_content_file_system_url_util and have other files use them

BUG=b:111816608
TEST=out/Default/unit_tests --gtest_filter="FileManagerPathUtilConvertUrlTest.*"

Change-Id: I4fac43a8f2f1c48389e275dfe56f7bb39efd6ef9
Reviewed-on: https://chromium-review.googlesource.com/c/1295976Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Satoshi Niwa <niwa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602206}