[tools] Update gm.py to re-run mksnapshot in gdb

It seems that the output on snapshot failure has changed, so failed
snapshots were no longer being automatically re-run in gdb.

Change-Id: I321a6055b5683db31d215b9c30827f4badca3df7
Reviewed-on: https://chromium-review.googlesource.com/1066058Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53269}

tools/dev/gm.py

@@ -246,7 +246,7 @@ class Config(object):
 
     return_code, output = _CallWithOutput("autoninja -C %s %s" %
                                           (path, targets))
-    if return_code != 0 and "FAILED: gen/snapshot.cc" in output:
+    if return_code != 0 and "FAILED: snapshot_blob.bin" in output:
       csa_trap = re.compile("Specify option( --csa-trap-on-node=[^ ]*)")
       match = csa_trap.search(output)
       extra_opt = match.group(1) if match else ""