Bug 1378410 - 2a. Expose set to moz.build sandbox; r=gps

To construct an empty set, we need to use the `set()` notation. In order
to do that, we need to expose `set` to the moz.build sandbox.

MozReview-Commit-ID: DMyKnF0FEx2

--HG--
extra : rebase_source : 5cfe8080ec333a1eca70cd3edba2aaaff6406820

build/docs/mozbuild-files.rst

@@ -34,10 +34,10 @@ The following properties make execution of ``moz.build`` files special:
    files.
 
 The limited subset of Python is actually an extremely limited subset.
-Only a few symbols from ``__builtins__`` are exposed. These include
-``True``, ``False``, and ``None``. Global functions like ``import``,
-``print``, and ``open`` aren't available. Without these, ``moz.build``
-files can do very little. *This is by design*.
+Only a few symbols from ``__builtin__`` are exposed. These include
+``True``, ``False``, ``None``, ``sorted``, ``int``, and ``set``. Global
+functions like ``import``, ``print``, and ``open`` aren't available.
+Without these, ``moz.build`` files can do very little. *This is by design*.
 
 The execution sandbox treats all ``UPPERCASE`` variables specially. Any
 ``UPPERCASE`` variable must be known to the sandbox before the script

python/mozbuild/mozbuild/frontend/sandbox.py

@@ -110,6 +110,7 @@ class Sandbox(dict):
         'True': True,
         'sorted': alphabetical_sorted,
         'int': int,
+        'set': set,
     })
 
     def __init__(self, context, builtins=None, finder=default_finder):