grit: utilize six.reraise

The syntax for raising a custom exception with all three args is
different between Python 2 & Python 3 to the point where we can't
even do it inline because an incompatible version throws syntax
errors during parsing.  Utilize six's reraise helper for it.

Bug: 983071
Test: `./grit/test_suite_all.py` passes
Change-Id: I2489dfe588c4c650661ece327c5d86e8754269ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700504Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686221}

tools/binary_size/supersize.pydeps

@@ -39,6 +39,7 @@
 ../grit/grit/tclib.py
 ../grit/grit/util.py
 ../grit/grit/xtb_reader.py
+../grit/third_party/six/__init__.py
 libsupersize/apkanalyzer.py
 libsupersize/ar.py
 libsupersize/archive.py

tools/grit/grit/gather/policy_json.py

@@ -10,6 +10,8 @@ from __future__ import print_function
 import json
 import sys
 
+import six
+
 from grit.gather import skeleton_gatherer
 from grit import util
 from grit import tclib
@@ -83,7 +85,7 @@ class PolicyJson(skeleton_gatherer.SkeletonGatherer):
       node = minidom.parseString(xml).childNodes[0]
     except ExpatError:
       reason = '''Input isn't valid XML (has < & > been escaped?): ''' + string
-      raise Exception, reason, sys.exc_info()[2]
+      six.reraise(Exception, reason, sys.exc_info()[2])
 
     for child in node.childNodes:
       if child.nodeType == minidom.Node.TEXT_NODE: