Fix observatory_tool so it works in mojo tree again

tools/observatory_tool.py

Index: tools/observatory_tool.py
diff --git a/tools/observatory_tool.py b/tools/observatory_tool.py
index 062ad1c24f40a5582834220a83f6e3e1c3d7cad1..68fcd86d9f2537b97f3959c7048e05b8baf0c807 100755
--- a/tools/observatory_tool.py
+++ b/tools/observatory_tool.py
@@ -44,6 +44,10 @@ def ProcessOptions(options, args):
   # Required options.
   if options.command is None or options.directory is None:
     return False
+
+  # Set a default value for pub_snapshot.
+  options.pub_snapshot = None
+
   # If we have a working pub executable, try and use that.
   # TODO(whesse): Drop the pub-executable option if it isn't used.
   if options.pub_executable is not None: