Update fieldtrial_testing_config.json entries for NIK/NAK partitioning

Removes `android_webview` from the HTTP Cache partitioning
fieldtrial_testing_config.json entry, since the feature hasn't launched
there (so it'd be better to have tests for that platform have the
feature disabled).

Also, adds comments mentioning that we rely on the
fieldtrial_testing_config.json entries for NSP and HTTP Cache
partitioning to enable the features for most types of tests.

Bug: None
Change-Id: I0548653fea799baca885d7f019f345f7f21a2cde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5697165
Reviewed-by: mmenke <mmenke@chromium.org>
Commit-Queue: Andrew Williams <awillia@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1327168}