[base::Time] Warn against storing floating point values

One such problematic pref at the moment is
kHintsFetcherHostsSuccessfullyFetched found while diagnosing
unexpected failures in
https://chromium-review.googlesource.com/c/chromium/src/+/1818219 which
makes microseconds-since-epoch a realistic (very large) number in
unit tests.

R=miu@chromium.org

Bug: 1004987, 1007427, 634507
Change-Id: I5ebf6a0ef46e2aea704ab5c54372ae77cbe64afb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1822840
Commit-Queue: Gabriel Charette <gab@chromium.org>
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Yuri Wiitala <miu@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699504}

base/time/time.h

@@ -121,6 +121,9 @@ class BASE_EXPORT TimeDelta {
   constexpr TimeDelta() : delta_(0) {}
 
   // Converts units of time to TimeDeltas.
+  // WARNING: Floating point arithmetic is such that FromXXXD(t.InXXXF()) may
+  // not precisely equal |t|. Hence, floating point values should not be used
+  // for storage.
   static constexpr TimeDelta FromDays(int days);
   static constexpr TimeDelta FromHours(int hours);
   static constexpr TimeDelta FromMinutes(int minutes);
@@ -208,6 +211,9 @@ class BASE_EXPORT TimeDelta {
   // towards zero, std::trunc() behavior). The InXYZFloored() versions round to
   // lesser integers (std::floor() behavior). The XYZRoundedUp() versions round
   // up to greater integers (std::ceil() behavior).
+  // WARNING: Floating point arithmetic is such that FromXXXD(t.InXXXF()) may
+  // not precisely equal |t|. Hence, floating point values should not be used
+  // for storage.
   int InDays() const;
   int InDaysFloored() const;
   int InHours() const;