Exclude test/assembly on Windows
Exclude test/assembly on Windows

The test isn't currently set up to work on Windows, but the exclusion was
previously being accomplished based on generator==msvs, rather than platform.
Exclude on Windows ninja too.

(Also fix inaccurate docstring)
Review URL: https://chromiumcodereview.appspot.com/9395080

csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp