Made the NavigationTracingObserver always be active whenever background tracing is active

This is needed so navigation tracing triggers can be used for any scenario, not just when --enable-navigation-tracing is set.

R=shatch,fmeawad

Review URL: https://codereview.chromium.org/1426803004

Cr-Commit-Position: refs/heads/master@{#357456}
7 files changed