Rename JankTracker to LayoutShiftTracker.

Also rename JankRegion to LayoutShiftRegion.

(There are still many references to "jank", which will be cleaned up in
follow-up patches.)

Bug: 963474
Change-Id: I33683bbd95dfea135f6e30215ddd09959913e8da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1644230
Commit-Queue: Steve Kobes <skobes@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670277}
19 files changed