[FCP++] Renaming paint-tracker as paint-timing-detector

The code base uses PaintTracker as the class to deal with FirstContentfulPaint++
logic. As it's specific for FCP++, we should give it a more specific name.
PaintTimingDetector will be more clear about its intention.

Bug: 869924
Change-Id: I194eeac2b40d1db3e719332e8a7cfe707a881b0e
Reviewed-on: https://chromium-review.googlesource.com/c/1348250
Commit-Queue: Liquan (Max) Gǔ <maxlg@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610489}
13 files changed