ozone: Use ozone_platform_dri to activate --ozone-platform-dri

This is to avoid interfering with other ozone-based builds.

BUG=chromium:362755
TEST=link_freon build & install
CQ-DEPEND=CL:199731

Change-Id: Ibb4f1b8cddbf36aaa194e94a11c335cf22694521
Reviewed-on: https://chromium-review.googlesource.com/199632
Reviewed-by: Albert Chaulk <achaulk@chromium.org>
Tested-by: Michael Spang <spang@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
diff --git a/session_manager_setup.sh b/session_manager_setup.sh
index 35d745f..31a185e 100755
--- a/session_manager_setup.sh
+++ b/session_manager_setup.sh
@@ -403,7 +403,7 @@
 
 # Ozone platform configuration
 OZONE_FLAGS=
-if use_flag_is_set ozone; then
+if use_flag_is_set ozone_platform_dri; then
   # TODO(spang): Use freon/chromeos platform, not DRI example platform.
   OZONE_FLAGS="$OZONE_FLAGS --ozone-platform=dri"