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}

tools_webrtc/mb/mb.py

@@ -36,6 +36,7 @@ SCRIPT_DIR = os.path.dirname(os.path.realpath(__file__))
 SRC_DIR = os.path.dirname(os.path.dirname(SCRIPT_DIR))
 sys.path = [os.path.join(SRC_DIR, 'build')] + sys.path
 
+import find_depot_tools
 import gn_helpers
 
 
@@ -1405,7 +1406,11 @@ class MetaBuildWrapper(object):
 
   def Build(self, target):
     build_dir = self.ToSrcRelPath(self.args.path[0])
-    ninja_cmd = ['ninja', '-C', build_dir]
+    ninja_cmd = [
+      os.path.join(find_depot_tools.DEPOT_TOOLS_PATH, 'ninja'),
+      '-C',
+      build_dir,
+    ]
     if self.args.jobs:
       ninja_cmd.extend(['-j', '%d' % self.args.jobs])
     ninja_cmd.append(target)