Use the Python shipped with depot_tools on Windows

third_party/python_26 was removed in
https://codereview.chromium.org/137023003.

script/bootstrap

@@ -25,6 +25,13 @@ def main():
         if result:
             return result
 
+    if sys.platform in ['win32', 'cygwin']:
+        if not os.path.isfile(os.path.join(DEPOT_TOOLS_DIR, 'python.bat')):
+            gclient = os.path.join(DEPOT_TOOLS_DIR, 'gclient.bat')
+            result = subprocess.call(['cmd.exe', '/c', gclient])
+            if result:
+                return result
+
     update = os.path.join(SOURCE_ROOT, 'script', 'update')
     return subprocess.call([sys.executable, update])
 

script/update

@@ -168,7 +168,7 @@ def install_win_tool_wrapper():
 def chromium_python_path():
     """Returns the win32 python shipped by Chromium."""
 
-    return os.path.join(SRC_DIR, 'third_party', 'python_26', 'python.exe')
+    return os.path.join(DEPOT_TOOLS_DIR, 'python_bin', 'python.exe')
 
 
 def mkdir_p(path):