Modernize PolicyProvider code.

- Use modern base::Value APIs.
- Stop using base::JSONReader::ReadDeprecated().
- Use range-based for-loops.
- Break DCHECK(cond1 && cond2) into DCHECK(cond1); DCHECK(cond2).
- Use constexpr when appropriate.
- Reformat some VLOG statements.

Change-Id: I8ba93963c5b107e5c1427f8a7c26f92513a8418d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2792739
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#867667}