Update chrome.dashboardPrivate API match

Bug: 937487
Change-Id: I2d985e28c56d4a7626e3d0c11a8be6d31499a66b
Reviewed-on: https://chromium-review.googlesource.com/c/1497631
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Christopher Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637049}