The more tests we add, the slower the bot becomes (and maybe annoy people of clang rolls). Need to find balance here.

Yeah, I agree balance is important, this is our current setup:
SAMPLE_TARGETS = ['base_unittests', 'url_unittests']
SAMPLE_FUZZER_TARGETS = ['pdfium_fuzzer']

So, maybe just add one more fuzzer target?

Good point, let's add something like third_party_re2_fuzzer. It's fairly small, very stable, but uses templates and other C++ stuff.

sg! I'll add third_party_re2_fuzzer. We'll add more targets on demand in the future if necessary.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/a23a27b7ec239bd277ccc787bb0c87ba6340cf85

commit a23a27b7ec239bd277ccc787bb0c87ba6340cf85
Author: Yuke Liao <liaoyuke@chromium.org>
Date: Wed May 23 19:14:19 2018

Coverage: Change sample targets to coverage tot builder

Now that the coverage tot builder is stable now, this CL changes the
sample targets used for testing.

Bug:  845547 
Change-Id: I0fc9e9dac7edea85ebc258092ccdc2fbad04b7d3
Reviewed-on: https://chromium-review.googlesource.com/1070314
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

[modify] https://crrev.com/a23a27b7ec239bd277ccc787bb0c87ba6340cf85/scripts/slave/README.recipes.md
[modify] https://crrev.com/a23a27b7ec239bd277ccc787bb0c87ba6340cf85/scripts/slave/recipes/chromium_clang_coverage_tot.expected/full_chromium_clang_ToTLinuxCoverage.json
[modify] https://crrev.com/a23a27b7ec239bd277ccc787bb0c87ba6340cf85/scripts/slave/recipes/chromium_clang_coverage_tot.expected/full_chromium_clang_ToTMacCoverage.json
[modify] https://crrev.com/a23a27b7ec239bd277ccc787bb0c87ba6340cf85/scripts/slave/recipes/chromium_clang_coverage_tot.py