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}
diff --git a/tools_webrtc/download_tools.py b/tools_webrtc/download_tools.py
index 5cc8de2..7324245 100755
--- a/tools_webrtc/download_tools.py
+++ b/tools_webrtc/download_tools.py
@@ -20,10 +20,11 @@
 
 SCRIPT_DIR = os.path.dirname(os.path.abspath(__file__))
 SRC_DIR = os.path.abspath(os.path.join(SCRIPT_DIR, os.pardir))
-DEPOT_TOOLS_DIR = os.path.join(SRC_DIR, 'third_party', 'depot_tools')
-sys.path.insert(0, DEPOT_TOOLS_DIR)
+sys.path.append(os.path.join(SRC_DIR, 'build'))
 
 
+import find_depot_tools
+find_depot_tools.add_depot_tools_to_path()
 import gclient_utils
 import subprocess2
 
@@ -35,7 +36,8 @@
   for path in directories:
     cmd = [
       sys.executable,
-      os.path.join(DEPOT_TOOLS_DIR, 'download_from_google_storage.py'),
+      os.path.join(find_depot_tools.DEPOT_TOOLS_PATH,
+                   'download_from_google_storage.py'),
       '--directory',
       '--num_threads=10',
       '--bucket', 'chrome-webrtc-resources',