Use CA certificates from standard location in Brillo.

Brillo images have all the CA certificates for the whole system in
/system/etc/security/cacerts. For update_engine, we would like to
accept only a sub-set of those, but we will start by accepting all
the installed certs.

Bug: 25089263
Test: `mm`, deployed on a -userdebug build and updated

Change-Id: Ib05e6ea1c5efb98a6320ce6288842e2277f67b1f
diff --git a/platform_constants.h b/platform_constants.h
index 189d134..310eed9 100644
--- a/platform_constants.h
+++ b/platform_constants.h
@@ -38,6 +38,11 @@
 // whole payload.
 extern const char kUpdatePayloadPublicKeyPath[];
 
+// Path to the directory containing all the SSL certificates accepted by
+// update_engine when sending requests to Omaha and the download server (if
+// HTTPS is used for that as well).
+extern const char kCACertificatesPath[];
+
 // Path to the file used to notify chrome about the deadline of the last omaha
 // response. Empty if not supported.
 extern const char kOmahaResponseDeadlineFile[];