The goal of this CL is to add trace events to track the lifetime and
state transitions of NavigationHandle. It will be very useful to be able
to debug navigations without debugger.

As part of this support, I'm adding a GetNameForLogging API to
NavigationThrottle, so tracing can attribute throttle checks to the
correct object.

BUG=

Review-Url: https://codereview.chromium.org/2830353003
Cr-Commit-Position: refs/heads/master@{#466667}