Check the context type in StopChromeTracing
DevToolsClientBackend.StopChromeTracing() tries to call console.time & console.timeEnd.
But in ServiceWorker telemetry tests, ServiceWorker may have already stopped by itself.
This is causing the test flakiness.
So this cl add checking if the context type is 'iframe', 'page' or 'webview'.
BUG=433943
TEST=./tools/perf/run_benchmark --browser-executable=~/chromium/src/out/Debug/chrome --also-run-disabled-tests service_worker.service_worker
Review URL: https://codereview.chromium.org/884573004
Cr-Commit-Position: refs/heads/master@{#317730}
2 files changed
