[code coverage] Enable 3 percent experiment for Mac OS.

Bug: 1024915
Change-Id: I681937537b65e12253dd122d521f49b193612654
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2049209Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Sajjad Mirza <sajjadm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747934}

infra/config/buckets/try.star

@@ -752,6 +752,7 @@ try_.chromium_mac_builder(
     name = 'mac-coverage-rel',
     use_clang_coverage = True,
     goma_jobs = goma.jobs.J150,
+    tryjob = try_.job(experiment_percentage = 3)
 )
 
 try_.chromium_mac_builder(

infra/config/generated/commit-queue.cfg

@@ -946,7 +946,7 @@ config_groups: <
       >
       builders: <
         name: "chromium/try/mac-coverage-rel"
-        includable_only: true
+        experiment_percentage: 3
       >
       builders: <
         name: "chromium/try/mac-dawn-rel"

infra/config/generated/cq-builders.md

@@ -337,6 +337,9 @@ as required builders.
 * [linux-ozone-x11-and-wayland-rel](https://ci.chromium.org/p/chromium/builders/try/linux-ozone-x11-and-wayland-rel) ([definition](https://cs.chromium.org/search?q=package:%5Echromium$+file:/cq.star$+-file:/beta/+-file:/stable/+linux-ozone-x11-and-wayland-rel)) ([matching builders](https://cs.chromium.org/search?q=+file:trybots.py+linux-ozone-x11-and-wayland-rel))
   * Experiment percentage: 1
 
+* [mac-coverage-rel](https://ci.chromium.org/p/chromium/builders/try/mac-coverage-rel) ([definition](https://cs.chromium.org/search?q=package:%5Echromium$+file:/cq.star$+-file:/beta/+-file:/stable/+mac-coverage-rel)) ([matching builders](https://cs.chromium.org/search?q=+file:trybots.py+mac-coverage-rel))
+  * Experiment percentage: 3
+
 * [win10_chromium_x64_coverage_rel_ng](https://ci.chromium.org/p/chromium/builders/try/win10_chromium_x64_coverage_rel_ng) ([definition](https://cs.chromium.org/search?q=package:%5Echromium$+file:/cq.star$+-file:/beta/+-file:/stable/+win10_chromium_x64_coverage_rel_ng)) ([matching builders](https://cs.chromium.org/search?q=+file:trybots.py+win10_chromium_x64_coverage_rel_ng))
   * Experiment percentage: 3