[CodeHeath] Remove DictionaryValue in BackgroundTracingConfig::FromDict

This removes DictionaryValue in BackgroundTracingConfig::FromDict and
related methods. BackgroundTracing::IntoDict will be converted in a
follow-up.

Notes on the conversion:
 - Tracing config has a nested list of dictionaries, so it was cleaner
 to convert this to use base::Value instead of base::Value::DictStorage,
 since using the latter would only work for one level since DictStorage
 is not a Value.
 - The public API was changed to pass base::Value&& instead of a
 pointer, in line with the guidance in base/values.h.
 - Changes FindBoolPath to FindBoolKey where the key was not a path
   (no '.' separator).

This also fixes a couple lint and presubmit errors in
content/browser/tracing/background_tracing_rule.cc

Bug: 1187061
Change-Id: I511e3fecabb0751b35eb9b0f3096f5633a3cefa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3131299


Reviewed-by: oysteine <oysteine@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Scott Haseley <shaseley@chromium.org>
Cr-Commit-Position: refs/heads/main@{#917747}