Move availability attribute pragmas.

Clean up availability attribute pragmas so that the pragmas only
surround standalone functions, and classes are just marked directly
with the attribute. This is the alternative to the previous workaround
of removing the pragma wherever there are lambdas we don't want the
attribute on. Marking classes with the attribute puts it on all of the
member functions, which is what we actually want.

Bug: 41487924, 345303691
Change-Id: I6732c18858431e720907edb5584bbd9825c2f8c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5622976
Commit-Queue: Amy Huang <akhuang@google.com>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313718}