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It is unclear what the rate of blob files missing are as a part of this
bug, and so add some metrics to try to get a handle on the scope of the
problem.

FilesystemProxyFileStreamReader is currently only used for
IndexedDBDataItemReader, but this creates a separate entry point and
metrics boolean in case this class gets reused for something other than
IndexedDB.

Bug: 1131151
Change-Id: I4d0610413534fb40edb95e321b6d58e92fdf23ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446889Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Auto-Submit: enne <enne@chromium.org>
Commit-Queue: enne <enne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814286}