We always want gclient_utils.FileRead() to return a text
string these days, but the existing code didn't work
right for Python2 if you called it explicitly with mode='r'.

R=ehmaldonado@chromium.org

Bug: 1210746
Change-Id: If540746f31308b671f9101334b5cd9848a7e9257
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2930626Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>