Add ninja wrappers to trigger DEPS ninja

This is the first step to deprecate ninja binaries in depot_tools.

depot_tools/{ninja, ninja.bat} will call ninja.py, which finds CIPD ninja under src/third_party in the current gclient source tree. https://crrev.com/c/3869740

If it fails to find, it does fallback to an existing ninja binary in depot_tools, until removing the references to the legacy ninja binaries.

# For those who see the deprecation message,
You need to install ninja using DEPS at your project. (https://crrev.com/c/3869740 is an example.)

If your project doesn't have DEPS, you need to install ninja and include it in PATH manually. e.g. sudo apt install ninja-build

Test:
Linux builder: https://ci.chromium.org/swarming/task/5e9cfc917aa3f110?server=chromium-swarm.appspot.com
Windows builder: https://ci.chromium.org/swarming/task/5e9d48885c460b10?server=chromium-swarm.appspot.com

Bug: 1340825
Change-Id: I70d1863d72ddfa65b87a62c9bde8ff63f7641d13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3925341
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>

ninja

@@ -4,37 +4,5 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-OS="$(uname -s)"
-THIS_DIR="$(dirname "${0}")"
-
-function print_help() {
-cat <<-EOF
-No prebuilt ninja binary was found for this system.
-Try building your own binary by doing:
-  cd ~
-  git clone https://github.com/ninja-build/ninja.git -b v1.8.2
-  cd ninja && ./configure.py --bootstrap
-Then add ~/ninja/ to your PATH.
-EOF
-}
-
-case "$OS" in
-  Linux)
-    MACHINE=$(uname -m)
-    case "$MACHINE" in
-      x86_64)
-        exec "${THIS_DIR}/ninja-linux64" "$@";;
-      *)
-        echo Unsupported architecture \($MACHINE\) -- unable to run ninja.
-        print_help
-        exit 1;;
-    esac
-    ;;
-  Darwin)    exec "${THIS_DIR}/ninja-mac" "$@";;
-  CYGWIN*)   exec cmd.exe /c $(cygpath -t windows $0).exe "$@";;
-  MINGW*)    cmd.exe //c $0.exe "$@";;
-  MSYS_NT*)  cmd.exe //c $0.exe "$@";;
-  *)         echo "Unsupported OS ${OS}"
-             print_help
-             exit 1;;
-esac
+base_dir=$(dirname "$0")
+PYTHONDONTWRITEBYTECODE=1 exec python3 "$base_dir/ninja.py" "$@"