Let gn_helpers.py handle unicode as str
Let gn_helpers.py handle unicode as str

For some reason, cros_chrome_sdk_unittests passes values as a unicode
object, instead of a str.

BUG=561142

Review URL: https://codereview.chromium.org/1851723002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384839}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk