Set the initial disk image size for mac packages.

Various stackoverflow answers suggest that setting an initial size can avoid errors while resizing the disk during the process of creating it.

python/servo/package_commands.py

@@ -274,7 +274,11 @@ def package(self, release=False, dev=False, android=None, debug=False, debugger=
                 os.remove(dmg_path)
 
             try:
-                subprocess.check_call(['hdiutil', 'create', '-volname', 'Servo', dmg_path, '-srcfolder', dir_to_dmg])
+                subprocess.check_call(['hdiutil', 'create',
+                                       '-volname', 'Servo',
+                                       '-megabytes', '900',
+                                       dmg_path,
+                                       '-srcfolder', dir_to_dmg])
             except subprocess.CalledProcessError as e:
                 print("Packaging MacOS dmg exited with return value %d" % e.returncode)
                 return e.returncode