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As a follow on to https://crrev.com/c/1248328, wrap conversions
between base::Time and the particular format used for persistence in
the quota database (int64_t) in methods, instead of inlining the
conversion steps at every point.

Bug: 889242
Change-Id: I8d5b7b6a6e7bf9ccf507cd8c3ad750dae9e30206
Reviewed-on: https://chromium-review.googlesource.com/1254744
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595589}