telemetry: Fix startup benchmarks so that all metrics get recorded.
PERF_SHERRIF: if startup benchmarks become red, this CL is to be reverted.
The lack of a page navigation was preventing the metric
Startup.FirstWebContents.NonEmptyPaint from being emitted. The addition of the
RAF logic check is to prevent theoretical race conditions - it may be overkill.
BUG=513364
CQ_EXTRA_TRYBOTS=tryserver.chromium.perf:linux_perf_bisect;tryserver.chromium.perf:mac_perf_bisect;tryserver.chromium.perf:android_nexus5_perf_bisect
Review URL: https://codereview.chromium.org/
1307203006
Cr-Commit-Position: refs/heads/master@{#347402}