Revert "Make autoninja.py work again w/ Python3 and MSYS/MinGW bash."

This reverts commit 9e2dfdcc.

Reason for revert:
build failure
https://chrome-internal-review.googlesource.com/c/goma/client/+/3683303
e.g. https://ci.chromium.org/ui/p/goma-client-internal/builders/try/Goma%20Trusty64%20Presubmit/b8852734992897102240/overview
https://logs.chromium.org/logs/goma-client-internal/buildbucket/cr-buildbucket.appspot.com/8852734992897102240/+/steps/build/0/stdout

 /b/s/w/ir/kitchen-checkout/depot_tools/autoninja: line 11: /b/s/w/ir/kitchen-checkout/depot_tools/python-bin/python3: No such file or directory
 /b/s/w/ir/kitchen-checkout/depot_tools/autoninja: line 20: /b/s/w/ir/kitchen-checkout/depot_tools/python-bin/python3: No such file or directory
 /b/s/w/ir/kitchen-checkout/depot_tools/autoninja: line 30: /b/s/w/ir/kitchen-checkout/depot_tools/python-bin/python3: No such file or directory

Original change's description:
> Make autoninja.py work again w/ Python3 and MSYS/MinGW bash.
>
> We switched autoninja.py to python3 in https://crrev.com/c/2752772
> but that assumes `python3` is in $PATH (which is likely true on
> Mac and Linux but less likely to be true on Windows, especially
> in MSYS/MinGW/git bash shells.
>
> It's safer to use the `python3` bundled into depot_tools in
> `$depot_tools/python-bin` and more hermetic to boot.
>
> Bug: 1187609
> Change-Id: I65efd1a2b87135ac4e25ba377309c3374cfda085
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2756889
> Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@google.com>

Bug: 1187609
Change-Id: I07b04d615624d3dc6bc071920ec97ed39583b64c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2758609
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>

autoninja

@@ -4,11 +4,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-depot_tools=$(dirname "$0")
-python3="$depot_tools/python-bin/python3"
-
 # Set unique build ID.
-AUTONINJA_BUILD_ID="$($python3 -c "import uuid; print(uuid.uuid4())")"
+AUTONINJA_BUILD_ID="$(python3 -c "import uuid; print(uuid.uuid4())")"
 export AUTONINJA_BUILD_ID
 
 if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
@@ -17,22 +14,23 @@ fi
 
 # Execute whatever is printed by autoninja.py.
 # Also print it to reassure that the right settings are being used.
-command=$($python3 "$depot_tools/autoninja.py" "$@")
+# Don't use python3 because it doesn't work in git bash on Windows.
+command=$(python "$(dirname -- "$0")/autoninja.py" "$@")
 if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
   echo "$command"
 fi
 if eval "$command"; then
   if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
-    $python3 "$depot_tools/post_build_ninja_summary.py" "$@"
+    python3 "$(dirname -- "$0")/post_build_ninja_summary.py" "$@"
   fi
 
   # Collect ninjalog from googler.
-  $python3 "$depot_tools/ninjalog_uploader_wrapper.py" --cmd $command
+  python3 "$(dirname -- "$0")/ninjalog_uploader_wrapper.py" --cmd $command
   exit
 fi
 
 # Collect ninjalog from googler.
-$python3 "$depot_tools/ninjalog_uploader_wrapper.py" --cmd $command
+python3 "$(dirname -- "$0")/ninjalog_uploader_wrapper.py" --cmd $command
 
 # Return an error code of 1 so that if a developer types:
 # "autoninja chrome && chrome" then chrome won't run if the build fails.