Report layout jank fractions to the WebLocalFrameClient.

This will allow the page_load_metrics component to observe the jank.

Also address nits in JankTracker::NotifyPrePaintFinished (empty viewport
check, DCHECK for non-zero jank fraction).

Bug: 581518
Change-Id: I22eab226dac31bd1b0ce284240bbca019ce49868
Reviewed-on: https://chromium-review.googlesource.com/c/1280616
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599912}