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https://crrev.com/c/2223973 attempted to fix this check to handle UTF-8
encoded files. However, it appears that Python2's open() will sometimes
return a bytestring for a file containing UTF-8 encoded text. encode()
will then assume the bytestring is encoded using the default 'ascii'
encoding, which then explodes upon seeing any byte with a high byte set
(e.g. any character that's not representable in 7-bit ASCII).

All mojom files in Chrome should be encoded as UTF-8. Since Python 2.7,
io.open() has identical functionality to Python 3's open(), so by using
that and specifying the encoding argument, the UTF-8 assumption can be
explicitly encoded. This makes it safe to unconditionally call encode()
to generate a bytestring to pass into the Mojo parser.

Bonus: this CL adds some additional gymanstics for exceptions to make it
easier to find the parse failure by using six.reraise().

Note: the original issue discussed in the CL description was fixed in a
different way by https://crrev.com/c/2225327, which uses codecs.open().
However, Python 3's open() is an alias for io.open(), which has somewhat
different behavior from codecs.open().

Change-Id: I86be3c5bc97c78078fe212162eb88e519180c3b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2226093Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774937}