On Windows, without quote, whole return value would be treated as
a command, and the execution would fail.
On Linux and Mac, without quote, if depot_tools is settled under
directories with ' ', command execution would fail because paths are
separated in a wrong way.

To make such a return value work on Linux and Mac, the shell script
started to use eval.

Bug: 902930
Change-Id: I9bb74585294af565988c0b844b6b113a5c685530
Reviewed-on: https://chromium-review.googlesource.com/c/1325249Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>