Also enforce that it is called on the main thread.

Change-Id: I827beefe625bebf6c03fce5220cfbbd68e807984
Reviewed-on: https://chromium-review.googlesource.com/591372Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47000}