[telemetry] Add TraceResult as return value from tracing, and placeholder trace_event_importer.

This is infrastructure for adding trace_event parsing to telemetry.

BUG=173327
NOTRY=True

Review URL: https://chromiumcodereview.appspot.com/12082091

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@179843 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed