Update GN prebuilt

This addresses TO-244.

Change-Id: Icf4cc7530341ad888d79838014a827210af71075
diff --git a/linux64/gn.sha1 b/linux64/gn.sha1
index af4f6ef..99fe0b0 100644
--- a/linux64/gn.sha1
+++ b/linux64/gn.sha1
@@ -1 +1 @@
-2d0a1468089dfda652090b5b43f44787be137475
+73a17cc71987e233d12b16e4f058f55cc164675e
diff --git a/mac/gn.sha1 b/mac/gn.sha1
index 5d89d36..cd89fd4 100644
--- a/mac/gn.sha1
+++ b/mac/gn.sha1
@@ -1 +1 @@
-256d6d6497d83c662100c4d2e22f849ad7e7bd99
+78fa88a52c89aeef92e016b5cf0b7458642591df