[DEPS] use python for mac_toolchain.py

c2193e7 switched download_nacl_toolchains.py
to vpython for crbug.com/784251, but it should switched mac_toolchain.py.
Revert c2193e7 and change mac_toolchain.py
to use python.

[email protected]

Bug: 784251
Change-Id: I32a56297aa58de8883f806a08ba3bd68da8180ab
No-Try: true
No-Presubmit: true
Reviewed-on: https://chromium-review.googlesource.com/769851
Commit-Queue: Nodir Turakulov <[email protected]>
Reviewed-by: Nodir Turakulov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#516449}

DEPS

@@ -735,7 +735,7 @@ hooks = [
     'pattern': '.',
     'condition': 'checkout_nacl',
     'action': [
-        'python',
+        'vpython',
         'src/build/download_nacl_toolchains.py',
         '--mode', 'nacl_core_sdk',
         'sync', '--extract',
@@ -764,7 +764,7 @@ hooks = [
     'name': 'mac_toolchain',
     'pattern': '.',
     'condition': 'checkout_ios or checkout_mac',
-    'action': ['vpython', 'src/build/mac_toolchain.py'],
+    'action': ['python', 'src/build/mac_toolchain.py'],
   },
   # Pull binutils for linux, enabled debug fission for faster linking /
   # debugging when used with clang on Ubuntu Precise.