Reland "printing: store margins in microns"

This is a reland of commit 8547c1b31123da74054b48a6859784fce9ac3dcd

components/printing/browser/print_to_pdf/pdf_print_utils.cc had logic
which clamped decimal part of mergins in points. Eg, if margins were
28.37 in points, the result was 28. That resulted in
`print_pages_params->params->content_size` to always have some size.
However, after changing `PrintSettings` to accept margins in microns and
converting those units, clamping stopped to happen in the same manner as
it used to be as units in microns are more precise (point per inch is 72
and microns per inch is 25400).

This results in different precision and if margins are too small, they
are not rounded in a way that they are in case of points. It's not 100%
clear what the reason is to do clamping. The only known fact is that it
fixed pdf tests in the headless backend.

Thus, to keep the workaround as it was, convert margins to points, clamp
them and only then convert the result to microns. That way, we preserve
the same result and wpt tests are happy.

Original change's description:
> printing: store margins in microns
>
> chrome.Printing will pass margins in microns as they'll eventually be
> converted to PWGs. Using points will result in loosing precision.
>
> Given that there are no strict restrictions in other places that set up
> PrintSettings, we can switch everything to microns.
>
> Bug: 316999874, 308709702
> Change-Id: I9d51c6a8413049315507043ccbb01d34d8b89be8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6354168
> Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
> Commit-Queue: Maksim Sisov <msisov@igalia.com>
> Reviewed-by: Peter Conn <peconn@chromium.org>
> Reviewed-by: Lei Zhang <thestig@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1437339}

Bug: 316999874, 308709702
Cq-Include-Trybots: luci.chromium.try:linux_chromium_dbg_ng
Change-Id: I3b73ce4f298dfed9253171679c25475f9e0e2614
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6396795
Reviewed-by: Peter Conn <peconn@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1442485}