Sync: Support omitting sensitive info in ConstructAboutInformation()

ConstructAboutInformation() gathers debugging information about the sync
state. It's employed both to power the sync-internals page - a debugging
page only accessible locally - as well as to compose logs sent to Google
in case of crashes (provided the user has agreed to this). For the
latter use case, the caller used to manually filter a section containing
PII, for privacy. Delegating this responsibility to the caller was bad
since the return value is quite "weakly typed" (base::DictionaryValue).
This meant that one might easily add new sensitive information to the
return value while forgetting to update the filter in the caller.

In this CL we transfer this responsibility to the method, by exposing
an include_sensitive_data parameter. Additionally, we now force the
constructor of a "section" to specify whether it is sensitive or not.
This last part makes the code more privacy-aware, which should make it
less likely to accidentally leak any data (even if this solution isn't
bullet-proof).

Bug: 973820
Change-Id: I99138a741c26755e138ac0a60dd08815aa90a10c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2467939
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816616}