Use helper macros to define base::Features in //components/page_load_metrics/browser

This allows:
- features to be defined with a consistent set of qualifiers, and for
  that set of qualifiers to be updated over time as appropriate.
- better PRESUBMIT checks to ensure that base::Features are not defined
  in headers.
- simplifies things for scripts trying to extract feature definitions
  out of C++ code.

The primary CL was generated using a script that automatically rewrites
base::Feature declarations and definitions to the macro form. Changes to
any files with known incompatibilities with the macros (base::Features
without static storage duration and base::Features declared as static
class members) were then fully reverted; those changes will be manually
handled in followups.

This CL was uploaded by git cl split.

R=sullivan@chromium.org

Bug: 1364289
Change-Id: Ie5fc344a1c12f2ea8860540e96ec503560c9ea22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3917005
Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1051049}