[FCP++] Implement Largest Text Paint and Last Text Paint

This CL implements Largest Text Paint and Last Text Paint.

Largest Text Paint timing measures how long to paint the largest text
element within viewport from navigation start, intending to proxy
main visible content delivery speed.

Last Text Paint timing measures how long to paint the last text within
viewport from navigation start, intending to proxy all visible
contents' delivery speed.

Bug: 869924
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I8b36fc1ec2e75998e00f6140446b326047c72140
Reviewed-on: https://chromium-review.googlesource.com/1158853
Commit-Queue: Liquan (Max) Gǔ <maxlg@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591100}
12 files changed