Chromium: make use of new get_files_affected_by_patch.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.blink",
       "--builder",
       "linux_blink_dbg",
@@ skipping 215 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 187 matching lines @@
       "@@@STEP_LINK@layout_test_results@https://storage.googleapis.com/chromium-layout-test-archives/linux_blink_dbg/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/linux_blink_dbg/571/layout-test-results.zip@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]