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Supersize tiger viewer until now used just a regular fetch since it only
dealt with public files. This cl allows it to use Google Cloud Storage'
json api to access private files which the user has access to.

Bug: 1048299
Change-Id: If4f4015efddea771c0de9fa36193d861a72902e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088639
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747419}