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from |double| to |base::Optional<base::Time>|.

This CL is the first step to stop using |double| for file timestamps in Blink.

Though base::Time can be an is_null() value, we apply
base::Optional<base::Time> instead of just base::Time because:

 - In JavaScript, the null base::Time, which represents
   1601-01-01 00:00 UTC, is a valid time. We have to use
   base::Optional<base::Time> in some parts of Blink.

 - data_element.mojom applies |mojo_base.mojom.Time?| for file timestamps,
  and it is mapped to |base::Optional<base::Time>| in C++.

This CL should have no user-visible behavior changes.

Bug: 988343
Change-Id: I91f592b31cf8e61053969530fafc689eea8ff538
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930756
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718489}