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There have been a number of reports about the storage pressure
notification being shown, even when there is an absence of
storage pressure.

This is due to a numerical precision issue, where division of two
ints would almost always return 0. This 0 meant that a condition
that checked for storage pressure would always pass, and QuotaManger
would fire storage pressure notifications for all write errors,
regardless of disk availability.

This change removes the division operation entirely with a little
trick:

 - Given percent_available = 100 * available_space / total,
 - percent_available < kStoragePressureThresholdRatio  is equivalent
     to (substitution):
 - 100 * available_space / total < kStoragePressureThresholdRatio
     is equivalent to (multiply both sides by total):
 - 100 * available_space < kStoragePressureThresholdRatio * total

TBR=pwnall@chromium.org

Bug: 1127237
Change-Id: I79c5ff47f27824d25ff8e47dc4aed859c8d3cd7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412758Reviewed-by: Jarryd Goodman <jarrydg@chromium.org>
Commit-Queue: Jarryd Goodman <jarrydg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807356}