[AFDO] Turn AFDO on by default for Linux official builds

This CL turns AFDO on for official builds of Linux. This appears to be a
<0.5% binary size regression, and should grant a pretty solid
performance gain.

This CL is also expected to increase build times of official builds by
around 12% (wall and CPU time). A memory usage increase of around
210-225MB per TU is also expected.

Depends on install-build-deps.sh change in r557698; it was re-run on
bots in crbug.com/841983, and devs were asked to update in "FYI: Linux
builds of Chromium need to run install-build-deps.sh after #557698".

Bug: 805098, 821595, 837025
Test: See issue 821595 for perf reports.
Change-Id: I9eafad1d20b897e77566de423f29c11cc8c13e83
Reviewed-on: https://chromium-review.googlesource.com/1025132
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558806}

• DEPS[diff]
• build/config/compiler/BUILD.gn[diff]