Add placeholder to insert sign-in token for testing

A subsequent test will reference this new #define.
Manually adding this define will override the bearer
authorization token used in requests, so that we can
simulate a signed-in account when running a test.

Bug: 1141619
Change-Id: I4fb34af8367a91fb09a7f2b8096aec94094d9bb6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518278


Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824493}