[Py3] Get blink_gc_plugin Py3 ready

Proper fix for process-graph.py (changes were reverted in
https://crrev.com/c/2266652).

Bug: chromium:941669
Change-Id: I38242be26371246a33a88630a1480ce8446b7680
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2267737
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783094}

tools/clang/blink_gc_plugin/process-graph.py

@@ -4,9 +4,13 @@
 # found in the LICENSE file.
 
 from __future__ import print_function
-from StringIO import StringIO
 import argparse, os, sys, json, subprocess, pickle
 
+try:
+  from StringIO import StringIO  # Python 2
+except:
+  from io import StringIO
+
 parser = argparse.ArgumentParser(
   description =
     "Process the Blink points-to graph generated by the Blink GC plugin.")