[ios] Convert time duration constants to base::TimeDelta

Using base::TimeDelta allow to specify the time unit when defining
the constant without resorting to putting the unit in the variable
name.

This also helps converting between different units with the helper
methods (.InSecondsF(), .InNanosecondsF(), ...).

This CL converts //ios/chrome/browser/upgrade.

Bug: none
Change-Id: I99759bfc8a83021bdcb80be4e53308b5366f9aec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3934897
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1055725}