When people execute autoninja, I believe people would expect ninja
in the autoninja directory would be used.  However, the original code
find ninja from PATH, and ninja in other directory could be used.
People usually do not notice this because having depot_tools in
PATH is recommended for Chromium developers.  However, in some
environments, an old version ninja is pre-installed, and unexpected
version ninja could be used upon PATH environment.

Also, in most of bots, depot_tools directory is not included in PATH.
Autoninja execution would fail there because the system cannot find
ninja.

Bug: b/77176746
Change-Id: Iad8bd952dc1e34a9d303fd5b493c555156369a17
Reviewed-on: https://chromium-review.googlesource.com/c/1319489Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@chromium.org>
Reviewed-by: Shinya Kawanaka <shinyak@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Auto-Submit: Yoshisato Yanagisawa <yyanagisawa@chromium.org>