Context: MANIFEST.json is used to track metadata about Web Platform Tests. It can be updated quickly while running the tests if it's already mostly-generated, but generating it from scratch is slow.

Many little changes cause changes in MANIFEST.json, which developers don't directly care about, and so ignoring it will make updating tests simpler.

See discussion in  bug 683485 .

Proposed solution:

 - Put MANIFEST.json in Google Storage
 - Add a gclient hook to download it from Google Storage
 - Add MANIFEST.json to .gitignore
 - Update the stored copy regularly (e.g. on import)
 - Update MANIFEST.json locally when running tests ( bug 666957 )