[Telemetry] Improve _GetProcJiffies operation with using grep.

[Telemetry] Improve _GetProcJiffies operation with using grep.

Previously, telemetry read the whole '/proc/timer_list' file just to parse the
first occurence of "jiffies". In case the file is big and telemetry is run
against remote device, this can create a big delay for transferring the file
to host device.

This patch modify it so that telemetry just use
'grep jiffies /proc/timer_list' to get the only lines that contain 'jiffies'.

Context: https://groups.google.com/a/chromium.org/forum/#!topic/telemetry/SblQqF0tHbc

Try job runs:
./tools/perf/run_benchmark --browser=trybot-all-android page_cycler.top_10_mobile --also-run-disabled-tests
https://codereview.chromium.org/1100233003

./tools/perf/run_benchmark --browser=trybot-all-mac page_cycler.intl_ja_zh
https://codereview.chromium.org/1106833004

./tools/perf/run_benchmark --browser=trybot-all-linux page_cycler.intl_ja_zh
https://codereview.chromium.org/1102673005

Review URL: https://codereview.chromium.org/1106043003

Cr-Commit-Position: refs/heads/master@{#326961}