IWYU: Remove base/command_line.h include from field_trial.h.

This removes the command line include from field_trial.h and
updates files that need that header to include it explicitly.

One exception is that browser_test_base.h is changed to
have an include for base/command_line.h instead of a forward
declaration, as many browser tests override functions that
take a base::CommandLine argument, in order to manipulate it.
It seems more reasonable to have this header have an
include for the file, given it provides an API for tests to
override that takes a CommandLine.

However, if desired, a follow-up CL can be created to switch
it back to a forward declaration and fix any compilation
problems that introduces.

Bug: 429365675
Change-Id: Ic190cc1e92613dd6b619b431f17f47e30465ed0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6885575
Auto-Submit: Alexei Svitkine <asvitkine@chromium.org>
Owners-Override: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1506797}