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/android/build_aar.py b/tools_webrtc/android/build_aar.py
index c86d266..0ddca2f 100755
--- a/tools_webrtc/android/build_aar.py
+++ b/tools_webrtc/android/build_aar.py
@@ -35,6 +35,7 @@
 
 
 SCRIPT_DIR = os.path.dirname(os.path.realpath(sys.argv[0]))
+SRC_DIR = os.path.normpath(os.path.join(SCRIPT_DIR, os.pardir, os.pardir))
 DEFAULT_ARCHS = ['armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64']
 NEEDED_SO_FILES = ['libjingle_peerconnection_so.so']
 JAR_FILE = 'lib.java/sdk/android/libwebrtc.jar'
@@ -47,6 +48,10 @@
 sys.path.append(os.path.join(SCRIPT_DIR, '..', 'libs'))
 from generate_licenses import LicenseBuilder
 
+sys.path.append(os.path.join(SRC_DIR, 'build'))
+import find_depot_tools
+
+
 
 def _ParseArgs():
   parser = argparse.ArgumentParser(description='libwebrtc.aar generator.')
@@ -66,14 +71,16 @@
 
 
 def _RunGN(args):
-  cmd = ['gn']
+  cmd = [sys.executable,
+         os.path.join(find_depot_tools.DEPOT_TOOLS_PATH, 'gn.py')]
   cmd.extend(args)
   logging.debug('Running: %r', cmd)
   subprocess.check_call(cmd)
 
 
 def _RunNinja(output_directory, args):
-  cmd = ['ninja', '-C', output_directory]
+  cmd = [os.path.join(find_depot_tools.DEPOT_TOOLS_PATH, 'ninja'),
+         '-C', output_directory]
   cmd.extend(args)
   logging.debug('Running: %r', cmd)
   subprocess.check_call(cmd)