The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7

commit aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7
Author: Jered Gray <jegray@google.com>
Date: Fri Dec 21 20:40:51 2018

Remove legacy support for top-level hint optimizations

Clients, starting at M-72, no longer support top-level hint
optimizations for the existing optimization types, so the code
supporting them is being removed.

The server may add new top-level optimization types in the future,
so the proto field is not being deprecated.

Bug:  917189 

Change-Id: Ib6a1bb96958ff96dfd65921d9afd4b8bac8bf39c
Reviewed-on: https://chromium-review.googlesource.com/c/1387950
Commit-Queue: Jered Gray <jegray@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Doug Arnett <dougarnett@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618585}
[modify] https://crrev.com/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7/components/optimization_guide/proto/hints.proto
[modify] https://crrev.com/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7/components/previews/content/previews_hints.cc
[modify] https://crrev.com/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7/components/previews/content/previews_hints.h
[modify] https://crrev.com/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7/components/previews/content/previews_optimization_guide_unittest.cc
[modify] https://crrev.com/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7/components/previews/core/previews_experiments.cc
[modify] https://crrev.com/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7/components/previews/core/previews_experiments.h
[modify] https://crrev.com/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7/components/previews/core/previews_features.cc
[modify] https://crrev.com/aa17b101c2ec8ec3eab6ab08c60b6e6883c505a7/components/previews/core/previews_features.h