Update scripts to use DEPS-pinned depot_tools

Always use gn.py in depot_tools instead of just gn.
The https://cs.chromium.org/chromium/src/build/find_depot_tools.py
is looking up the DEPS-pinned copy in third_party/depot_tools
and adds it to the path when add_depot_tools_to_path() is called.

Similar use:
 

https: //cs.chromium.org/search/?q=%22find_depot_tools.add_depot_tools_to_path()%22&sq=package:chromium&type=cs
Bug: webrtc:8393
Change-Id: I3cfa3d96b4d0f60e8099e556876bc94340b1bbb5
Reviewed-on: https://webrtc-review.googlesource.com/12540
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@google.com>
Commit-Queue: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20333}

examples/androidtests/gradle_project_test.py

@@ -60,10 +60,15 @@ def main():
   project_dir = os.path.abspath(project_dir)
 
   try:
+    env = os.environ.copy()
+    env['PATH'] = os.pathsep.join([
+        os.path.join(SRC_DIR, 'third_party', 'depot_tools'), env.get('PATH', '')
+    ])
     _RunCommand([GENERATE_GRADLE_SCRIPT, '--output-directory', output_dir,
         '--target', '//examples:AppRTCMobile',
         '--project-dir', project_dir,
-        '--use-gradle-process-resources', '--split-projects', '--canary'])
+        '--use-gradle-process-resources', '--split-projects', '--canary'],
+        env=env)
     _RunCommand([GRADLEW_BIN, 'assembleDebug'], project_dir)
   finally:
     # Do not delete temporary directory if user specified it manually.