GYP: fix the list of files output by the mojom generator.

The .html output of the generator was changed to .sky while a pervious CL was
in flight: https://codereview.chromium.org/717873004/
Because ninja was expecting a .html file that was never produced, it would
always re-run the code generation step.

BUG=https://code.google.com/p/chromium/issues/detail?id=401761
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/732793002