Add Nearby Messages API to proguard config

The Physical Web feature of Chrome can take advantage of the Nearby
API, so we need to keep proguard from removing it.

BUG=623638

Change-Id: I6e9d736d5b36396762aec3415a56149a0137647f
Reviewed-on: https://chromium-review.googlesource.com/356196
Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Nicolas Dossou-Gbété <dgn@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Tested-by: Ross McIlroy <rmcilroy@chromium.org>