Previously, it returned bytes if the file did not contain valid Unicode data,
and a Unicode string otherwise.
It is confusing to reason about such a function, and no current caller needs
bytes data AFAICT, so make FileRead always return Unicode strings.

Bug: 1009814
Change-Id: I89dd1935e5d4fcaf9af71585b85bda6c47695950
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1880013Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>