Make json5_generator.Writer always take a list of files.

The json5_generator.Writer constructor has a convenient interface where
you can specify either the name of a single file you want to read from,
or a list of files. However, this requires a branch in Writer that
checks if the argument is a string or a list, which is not Pythonic.

This patch makes Writer always take a list of files.

BUG=628043

Review-Url: https://codereview.chromium.org/2815773003
Cr-Commit-Position: refs/heads/master@{#466175}
2 files changed