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This may hopefully make the next person seeing reports of strange 1-byte
range requests for previously truncated files less flabbergasted.

(And also documented some fields at high-level).

Bug: 820862
Change-Id: I3bbb8fc46c2c5a015bf8669165add631df289486
Reviewed-on: https://chromium-review.googlesource.com/964981Reviewed-by: Helen Li <xunjieli@chromium.org>
Commit-Queue: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543907}