[Telemetry] Make chrome_tracing_agent state more resilient to tracing exception.
Because ChromeTracingAgent class has global states, when any
exception was raised inside StopTracing method, the active tracing
state of this class is not set to False, making subsequent unittest that
try to StartTracing fails because of this.
Long term fix of this depends on crbug.com/317481 be fixed so that
ChromeTracingAgent no longer have global state.
BUG=459807
Review URL: https://codereview.chromium.org/
997193002
Cr-Commit-Position: refs/heads/master@{#320799}