Handle main resource timings in DocumentLoader

This is a preparation to not using WebURLLoader
for main resource. See step 6.d in the doc linked
to the bug.

Bug: 855189
Change-Id: I4464daabca68a24bb5272481af379c91b4d67d5e
Reviewed-on: https://chromium-review.googlesource.com/c/1392498
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620183}
12 files changed