Upgrade bazlets to latest stable-2.15 to build with 2.15.12 API
Upgrade bazlets to latest stable-2.15 to build with 2.15.12 API

Change-Id: Ie75f20a9f72299b6d16951c1ed7bfcc21dce3586

GerritCodeReview/plugins_quota,GerritCodeReview/plugins_quota,GerritCodeReview/plugins_quota