Adds metrics that track when sessions start and stop, and why; and how
long they lasted; and coarsely-grained stats about codecs/profiles that
were being used. This will help us track that the system is performing
as expected in upcoming trials.

This change also added a lot of missing error-reporting structure, to
properly account for all possible start and end triggers throughout the
implementation. In some places, small tweaks to shutdown-on-error logic
were made.

Furthermore, a minor change to the fullscreen exit control logic was
made (since the histograms revealed a subtle flaw that would prevent
exiting remoting in some cases where the user's action requested it).

BUG=643964
TEST=Manual testing of several different media remoting scenarios, and checking chrome://histograms to confirm correct measurement values and event tracking.

Review-Url: https://codereview.chromium.org/2631993002
Cr-Commit-Position: refs/heads/master@{#444230}