Fix process-graph.py after #698787

TBR=thakis

Bug: 1007195
Change-Id: I8280152158f63cfa2e3ad550eca726036b578506
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1820801Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699274}

tools/clang/blink_gc_plugin/process-graph.py

@@ -3,6 +3,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+from __future__ import print_function
 import argparse, os, sys, json, subprocess, pickle, StringIO
 
 parser = argparse.ArgumentParser(