[BrowserSwitcher] Apply default browser even when pref is not managed

Previously, the code that would set IE/Safari (based on platform) as the
default browser would not apply correctly because of a regression.  This
CL applies the rules again, even when the AlternativeBrowserPath policy
is not set.

Bug: 911779
Change-Id: I79c758b9295f67682934bbc299cfaa544b1a30d0
Reviewed-on: https://chromium-review.googlesource.com/c/1372411
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617024}