Trace where timers were scheduled in Blink

This required adding a TraceLocation class
modeled after base::Location in Chromium.

This makes TraceLocation a required parameter
to all Timer::start* calls.

BUG=350592
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/189833009

git-svn-id: svn://svn.chromium.org/blink/trunk@168858 bbb929c8-8fbe-4397-9dbb-9b2b20218538