Use TimeDelta in FirstMeaningfulPaintDetector

This changes the |network2_quiet_window_seconds_| and
|network0_quiet_window_seconds_| fields from double to TimeDelta,
renaming them to not have 'seconds' in the name (replaced by 'timeout'.)

Adjust the unit test, and replace AdvanceClockSeconds with AdvanceClock
throughout.

Add a new TestingPlatformSupportWithMockScheduler::RunForPeriod,
mimicking the setup for AdvanceClock{,Seconds}.

Bug: 763980
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9316759a30c6fbe3fb4d7ea15106d6c47366ba33
Reviewed-on: https://chromium-review.googlesource.com/1127779
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573298}
5 files changed