Scalable Notifiers: add IsBrowserContextEligible

Since the initialization of the scalable IPH service is now dependent
on the initialization of some other services, add
`IsBrowserContextEligible` as a substitute check for scalable IPH
availability. If the browser context is eligible, we will assume that
the service will also become available and proceed (i.e. show the
Welcome Tips page in the Explore app after OOBE).

Test: tested manually, existing tests
Bug: b/300708368
Change-Id: I0c33c16e277fc3e5a160e10d1fe505d9b328800c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4875454
Reviewed-by: Callistus Tan <callistus@google.com>
Commit-Queue: Angela Xiao <angelaxiao@chromium.org>
Reviewed-by: Yuki Awano <yawano@google.com>
Cr-Commit-Position: refs/heads/main@{#1198108}