This flag allows the user to specify a file which contains a JSON list of dicts to set for the "changes" property when scheduling the build.

BUG=493885

Review URL: https://codereview.chromium.org/1199963004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295788 0039d316-1c4b-4281-b951-d872f2087c98