Are they testers or builders? Should we have linux-code-coverage-builder and linux-code-coverage-tester pairs instead?

These are meant to be builders-only at the moment. Later on we'll set up
testers.

I'd suggest targeting at Linux for now and using more self-explaining names.

I think you can just reuse:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-code-coverage-generation and
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/mac-code-coverage-generation

I named the builder as linux-code-coverage-generation, but since we're going to pipeline the process, so I think it's better to rename to linux-code-coverage-builder.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/6287d83e5f0062d8a2c8e87aa59357f12466779f

commit 6287d83e5f0062d8a2c8e87aa59357f12466779f
Author: Roberto Carrillo <robertocn@google.com>
Date: Wed Oct 03 23:05:46 2018

Add linux-code-coverage-generation to bot_db

Bug:  891532 
Change-Id: Iacc1377381cf0d8bf9e9225cb00786feafda9940
Reviewed-on: https://chromium-review.googlesource.com/c/1258608
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>

[modify] https://crrev.com/6287d83e5f0062d8a2c8e87aa59357f12466779f/scripts/slave/recipe_modules/chromium_tests/chromium_fyi.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1062eb7a0a9a2e8f1f27592f877033462f24c74d

commit 1062eb7a0a9a2e8f1f27592f877033462f24c74d
Author: Roberto Carrillo <robertocn@google.com>
Date: Thu Oct 04 05:33:10 2018

[Code coverage] Configure linux code coverage builder.

This change should make chromium.fyi/linux-code-coverage-generation
build one target with code coverage instrumentation and isolate it using
the chromium recipe.

This depends on change http://crrev.com/c/1258608

Tested locally:

~/chromium/src/tools$ ./mb/mb.py lookup -m chromium.fyi -b \
linux-code-coverage-generation

Writing """\
is_clang = true
is_component_build = false
is_debug = false
strip_absolute_paths_from_debug_symbols = true
symbol_level = 0
use_clang_coverage = true
use_goma = true
""" to _path_/args.gn.

R=mmoroz,stgao,liaoyuke

Bug: chromium: 891532
Change-Id: I5afdd45d149fad0f62b8a6b8f021c99d0217134c
Reviewed-on: https://chromium-review.googlesource.com/c/1255590
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596514}
[modify] https://crrev.com/1062eb7a0a9a2e8f1f27592f877033462f24c74d/infra/config/global/cr-buildbucket.cfg
[modify] https://crrev.com/1062eb7a0a9a2e8f1f27592f877033462f24c74d/testing/buildbot/chromium.fyi.json
[modify] https://crrev.com/1062eb7a0a9a2e8f1f27592f877033462f24c74d/testing/buildbot/test_suites.pyl
[modify] https://crrev.com/1062eb7a0a9a2e8f1f27592f877033462f24c74d/testing/buildbot/waterfalls.pyl
[modify] https://crrev.com/1062eb7a0a9a2e8f1f27592f877033462f24c74d/tools/mb/mb_config.pyl