[WebUI] Add chrome.timeTicks.nowInMicroseconds() to WebUI

Add chrome.timeTicks.nowInMicroseconds() to WebUI. It returns the
microseconds since the origin of TimeTicks as a BigInt. It is equivalent
of TimeTicks::Now().since_origin().InMicroseconds() in C++.

This function will facilitate calculating delta between TimeTicks
captured in browser and WebUI. This is currently not possible because
time units in JS (performance.now, Date) use different origins from
TimeTicks.

See more discussion in doc: go/timeticks-in-webui

Note: We can't expose TimeTicks's raw value as chrome.timeTicks.now().
because a) it will involve the banned TimeTicks::ToInternalValue(),
b) TimeTicks difference in JS becomes meaningless because it has no unit.

Bug: 1269417
Change-Id: I18a1868ce1c1f9498660c7298b038907d0fef25a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3331615


Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958422}