Chromium: make use of new get_files_affected_by_patch.

scripts/slave/recipes/chromium_trybot.expected/swarming_test_failure.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.chromium.linux",
       "--builder",
       "linux_chromium_rel_ng",
@@ skipping 239 matching lines @@
     "cmd": [
       "git",
       "diff",
       "--cached",
       "--name-only"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "name": "git diff to analyze patch",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
-      "@@@STEP_LOG_LINE@files@foo.cc@@@",
+      "@@@STEP_LOG_LINE@files@src/foo.cc@@@",
       "@@@STEP_LOG_END@files@@@",
       "@@@SET_BUILD_PROPERTY@subproject_tag@\"chromium\"@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]/src/testing/buildbot/trybot_analyze_config.json",
@@ skipping 697 matching lines @@
       "@@@SET_BUILD_PROPERTY@failure_type@\"TEST_FAILURE\"@@@",
       "@@@SET_BUILD_PROPERTY@failure_hash@\"2c817ab2d4394c69c9b8a0698b954936d9e1fb76\"@@@"
     ]
   },
   {
     "name": "$result",
     "reason": "1 out of 1 aggregated steps failed. Failures: Step('gl_tests (retry summary)') failed with return_code 1",
     "status_code": 1
   }
 ]