diagnose_bloat.py: Add default value for tool_prefix.

Without it the --cloud option is broken.

Change-Id: I86c4a6c1975904e3d1944cb4daf50c6cbd833cd5
Reviewed-on: https://chromium-review.googlesource.com/1115502Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570512}

tools/binary_size/diagnose_bloat.py

@@ -359,7 +359,7 @@ class _BuildArchive(object):
     self._slow_options = slow_options
     self._save_unstripped = save_unstripped
 
-  def ArchiveBuildResults(self, supersize_path, tool_prefix):
+  def ArchiveBuildResults(self, supersize_path, tool_prefix=None):
     """Save build artifacts necessary for diffing."""
     logging.info('Saving build results to: %s', self.dir)
     _EnsureDirsExist(self.dir)