Since the swiftshader libraries are loaded at runtime
by chromium, the correct way to add swiftshader as a
dependency is to use 'loadable_module' for mac and
'data_deps' for other platforms.

BUG=31645700

Change-Id: Ic393a4132fbfc44e71247fc0294cca2cb57d35c8
Reviewed-on: https://swiftshader-review.googlesource.com/7217Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>