Use helper macros to define base::Features in //chromeos

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.

One manual fix was required for a feature that specified the export
attribute on the definition rather than the declaration. While legal, it
is far more common to specify the export attribute on the declaration.

Bug: 1364289
Change-Id: I5f43c979e1b46313109ebdcc9199b804aeba92eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3916971
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1051318}

• chromeos/ash/components/chromebox_for_meetings/features.cc[diff]
• chromeos/ash/components/chromebox_for_meetings/features.h[diff]
• chromeos/ash/components/memory/kstaled.cc[diff]
• chromeos/ash/components/memory/kstaled.h[diff]
• chromeos/ash/components/memory/memory.cc[diff]
• chromeos/ash/components/memory/memory.h[diff]
• chromeos/ash/components/memory/swap_configuration.cc[diff]
• chromeos/ash/components/memory/swap_configuration.h[diff]
• chromeos/ash/components/memory/userspace_swap/userspace_swap.cc[diff]
• chromeos/ash/services/assistant/public/cpp/features.cc[diff]
• chromeos/ash/services/assistant/public/cpp/features.h[diff]
• chromeos/ash/services/libassistant/service_controller.cc[diff]
• chromeos/constants/chromeos_features.cc[diff]
• chromeos/constants/chromeos_features.h[diff]
• chromeos/hugepage_text/hugepage_text.cc[diff]
• chromeos/hugepage_text/hugepage_text.h[diff]
• chromeos/system/core_scheduling.cc[diff]
• chromeos/ui/wm/features.cc[diff]
• chromeos/ui/wm/features.h[diff]