[code coverage] Set up optional trybot for code coverage.

This is a companion cl for:
https://chromium-review.googlesource.com/c/chromium/tools/build/+/1313151

R=liaoyuke,stgao,jbudorick

Change-Id: Ic0d4d3dc9ac92c30b9a8c5c0b018edf604fde750
Reviewed-on: https://chromium-review.googlesource.com/c/1313758Reviewed-by: Shuotao Gao <stgao@chromium.org>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Roberto Carrillo <robertocn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605459}

infra/config/global/cr-buildbucket.cfg

@@ -3060,6 +3060,11 @@ buckets {
     builders { mixins: "linux-try" name: "linux_chromium_dbg_ng" }
     builders { mixins: "linux-try" name: "linux_chromium_headless_rel" }
     builders { mixins: "linux-try" name: "linux_chromium_msan_rel_ng" }
+    builders {
+      mixins: "linux-try"
+      mixins: "goma-j150"
+      name: "linux-coverage-rel"
+    }
     builders {
       mixins: "linux-try"
       mixins: "goma-j150"

infra/config/global/luci-milo.cfg

@@ -4890,6 +4890,9 @@ consoles {
   builders {
     name: "buildbucket/luci.chromium.try/linux_chromium_component_updater"
   }
+  builders {
+    name: "buildbucket/luci.chromium.try/linux-coverage-rel"
+  }
   builders {
     name: "buildbucket/luci.chromium.try/linux_chromium_dbg_32_ng"
   }

tools/mb/mb_config.pyl

@@ -612,6 +612,7 @@
       'linux-blink-gen-property-trees': 'release_trybot',
       'linux-blink-heap-incremental-marking': 'debug_trybot_enable_blink_heap_incremental_marking',
       'linux-blink-heap-verification-try': 'release_trybot_enable_blink_heap_verification',
+      'linux-coverage-rel': 'clang_code_coverage_trybot',
       'linux-dcheck-off-rel': 'release_trybot_dcheck_off',
       'linux-goma-rbe-staging-rel': 'gpu_tests_release_trybot_no_symbols',
       'linux-gcc-rel': 'release_bot_x86_minimal_symbols_no_clang_cxx11',
@@ -1090,6 +1091,10 @@
       'release_bot', 'clang', 'use_clang_coverage', 'no_symbols',
     ],
 
+    'clang_code_coverage_trybot': [
+      'gpu_tests', 'use_dummy_lastchange', 'release_trybot', 'clang', 'use_clang_coverage', 'no_symbols',
+    ],
+
     'clang_release_bot_minimal_symbols_x86': [
       'clang', 'release_bot', 'minimal_symbols', 'x86',
     ],