Support Python 3 in jni_generator.py

The script still works with Python 2.
There are no intended behaviour changes.

Bug: chromium:941669
Change-Id: Iff075d8214ed60b17e8cd5a1c7f56ba60187675a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1596432
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Oleh Prypin <oprypin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656897}
4 files changed