Don't buffer stdout in DM

TBR=egdaniel@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1922223002

Review URL: https://codereview.chromium.org/1922223002
diff --git a/dm/DM.cpp b/dm/DM.cpp
index 586a4e7..e9b4585 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -1284,6 +1284,7 @@
 
 int dm_main();
 int dm_main() {
+    setbuf(stdout, nullptr);
     setup_crash_handler();
 
     if (FLAGS_verbose) {