Track PerformanceManager PageNodes in system profiles

https://crrev.com/c/4518422 disabled tracking of PageNodes for
WebContents in profiles that have Keyed Services disabled (such as
the profile picker page in the system profile). This was too
restrictive - PerformanceManager also tracks data that doesn't use
Keyed Services that should still be enabled.

This means that whenever SiteData is looked up for a PageNode,
SiteDataCacheFactory and SiteDataCacheFacadeFactory may not exist (if
only the system profile is open), and when they do exist,
GetDataCacheForBrowserContext() and GetInspectorForBrowserContext()
may return nullptr (if the browser context disabled Keyed Services).

Therefore this adds tests for nullptr in paths where Keyed Services
may be disabled, and explicit CHECK's in paths where they should
always be enabled. (Except in the SiteDataCacheFacade class, which
is a KeyedService itself and where CHECK'ing every call to
SiteDataCacheFacadeFactory::GetInstance() would be too verbose.)

R=grt

Bug: 1284664
Change-Id: If6684b8230482682ecb9e70d55e4953061f66309
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5443769
Commit-Queue: Joe Mason <joenotcharles@google.com>
Auto-Submit: Joe Mason <joenotcharles@google.com>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1285845}