[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 unit test code calling WaitUntilConditionOrTimeout
to use the new override taking the timeout as base::TimeDelta in
//ios/chrome/test/wpt.

Bug: none
Change-Id: I707d194348ebb4fd6da39a059a9cc2df877cb260
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4008642
Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1068548}