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>

proguard.flags

diff --git a/proguard.flags b/proguard.flags
index 1170153..ed50447 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -18,7 +18,9 @@
 -keep class com.google.android.gms.auth.*,             # Signin, chromoting
             com.google.android.gms.cast.*,             # Cast
             com.google.android.gms.gcm.*,              # GCM for CacheInvalidation
-            com.google.android.gms.iid.*               # GCM Registration for CacheInvalidation
+            com.google.android.gms.iid.*,              # GCM Registration for CacheInvalidation
+            com.google.android.gms.nearby.*,           # Physical Web
+            com.google.android.gms.nearby.messages.*   # Physical Web
       {
   *;
 }