Merge 104582 - Add support for an optional "requirements" section in extension/app manifests.

Merge 104582 - Add support for an optional "requirements" section in extension/app manifests.

The expected format is:

"requirements": {
  "requirement1": {
    // requirement details
  },
  "requiement2": {
    // requirement details
  },
  ...
}

Only validation and whitelisting for hosted apps is done, for now only the web
store will use requirements data.

BUG=99241
R=yoz@chromium.org

Review URL: http://codereview.chromium.org/8200015

TBR=mihaip@chromium.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=104912