I don't think this is necessary for M-52, so slotting for M-53. Devin, please comment if you believe otherwise.

SGTM. This is for the no-images experiment, which is targeted towards PLT not bytes. Since the latter is less noisy, it should be our first metric to check in stable expts. tl;dr not a blocker

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e71ee4f99e0b249722b20051ea2db80f4071d012

commit e71ee4f99e0b249722b20051ea2db80f4071d012
Author: rajendrant <rajendrant@chromium.org>
Date: Sat Jun 04 19:13:25 2016

Select precache resources based on field trial experiment group

Resource manifest will include bitset of the resources to be fetched
based on the fieldtrial experiment id.

BUG= 616675 

Review-Url: https://codereview.chromium.org/2037523002
Cr-Commit-Position: refs/heads/master@{#397935}

[modify] https://crrev.com/e71ee4f99e0b249722b20051ea2db80f4071d012/components/precache/content/precache_manager.cc
[modify] https://crrev.com/e71ee4f99e0b249722b20051ea2db80f4071d012/components/precache/core/precache_fetcher.cc
[modify] https://crrev.com/e71ee4f99e0b249722b20051ea2db80f4071d012/components/precache/core/precache_fetcher.h
[modify] https://crrev.com/e71ee4f99e0b249722b20051ea2db80f4071d012/components/precache/core/precache_fetcher_unittest.cc
[modify] https://crrev.com/e71ee4f99e0b249722b20051ea2db80f4071d012/components/precache/core/proto/precache.proto