Chromium: make use of new get_files_affected_by_patch.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 243 matching lines @@
     "cmd": [
       "RECIPE_PACKAGE_REPO[depot_tools]\\git.bat",
       "diff",
       "--cached",
       "--name-only"
     ],
     "cwd": "[SLAVE_BUILD]\\src",
     "name": "git diff to analyze patch",
     "stdout": "/path/to/tmp/",
     "~followup_annotations": [
-      "@@@STEP_LOG_LINE@files@third_party/WebKit/Source/core/dom/Element.cpp@@@",
+      "@@@STEP_LOG_LINE@files@src/third_party/WebKit/Source/core/dom/Element.cpp@@@",
       "@@@STEP_LOG_END@files@@@",
       "@@@SET_BUILD_PROPERTY@subproject_tag@\"blink\"@@@"
     ]
   },
   {
     "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 264 matching lines @@
       "Release"
     ],
     "name": "process_dumps"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]