Update base::TimeDelta::InMilliseconds to use constexpr

There are a lot of callers of base::TimeDelta::InMilliseconds
that seem like they'd benefit from this being constexpr, so
this CL makes that change.

The Chromium Binary Size check says this increases the Android
binary size by 12,403 bytes, which is below the 16 kilobytes per
CL threshold.

Bug: 761570
Change-Id: Ida7fbfa37d61476585846176d0674aa042ceffa9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4689945
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Andrew Williams <awillia@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1172431}