gsutil: use depot_tools.gsutil_py_path instead of path['depot_tools']

scripts/slave/recipes/webrtc/ios_api_framework.expected/build_ok.json

 [
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "name": "host and tools info",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br />OS X 1.2.3, Xcode 6.7.8 (5D342509a)@@@",
@@ skipping 104 matching lines @@
     ],
     "name": "zip archive",
     "stdin": "{\"entries\": [{\"path\": \"[SLAVE_BUILD]/src/out_ios_libs/WebRTC.framework\", \"type\": \"dir\"}, {\"path\": \"[SLAVE_BUILD]/src/out_ios_libs/WebRTC.framework.dSYM\", \"type\": \"dir\"}, {\"archive_name\": null, \"path\": \"[SLAVE_BUILD]/src/out_ios_libs/LICENSE.html\", \"type\": \"file\"}], \"output\": \"[SLAVE_BUILD]/webrtc_ios_api_framework.zip\", \"root\": \"[SLAVE_BUILD]/src/out_ios_libs\"}"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::gsutil]/resources/gsutil_wrapper.py",
       "--",
-      "[DEPOT_TOOLS]/gsutil.py",
+      "RECIPE_PACKAGE_REPO[depot_tools]/gsutil.py",
       "----",
       "cp",
       "-a",
       "public-read",
       "[SLAVE_BUILD]/webrtc_ios_api_framework.zip",
       "gs://chromium-webrtc/ios_api_framework/webrtc_ios_api_framework_170242.zip"
     ],
     "name": "gsutil upload",
     "~followup_annotations": [
       "@@@STEP_LINK@gsutil.upload@https://storage.googleapis.com/chromium-webrtc/ios_api_framework/webrtc_ios_api_framework_170242.zip@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]