Bug 969227 - Part 1: handle X-Backoff headers.
Bug 969227 - Part 2: add handleBackoff to TokenServerClientDelegate.
Bug 969227 - Part 3: handle Retry-After and X-Backoff in TokenServerClient.
Review comment: add SyncResponse.getContentType.
Review comment: rename invokeHandleBackoff to notifyBackoff.
Review comment: describing notifyBackoff/handleBackoff.
Bug 969227 - Handle X-Backoff headers. r=nalexander