Extracted Mako compilation to separate .py files.

This breaks out some of the parts on #10586, that should be easily mergeable. The idea would be to let you review & merge it first, and then I'll complete the other PR rebase off of this stuff.

components/style/generate_properties_rs.py

@@ -0,0 +1,17 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import os
+import sys
+
+from mako import exceptions
+from mako.template import Template
+
+try:
+    template = Template(open(os.environ['TEMPLATE'], 'rb').read(),
+                        input_encoding='utf8')
+    print(template.render(PRODUCT=os.environ['PRODUCT']).encode('utf8'))
+except:
+    sys.stderr.write(exceptions.text_error_template().render().encode('utf8'))
+    sys.exit(1)