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/+/2756889Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>

autoninja

@@ -4,8 +4,11 @@
 # 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
@@ -14,23 +17,22 @@ fi
 
 # Execute whatever is printed by autoninja.py.
 # Also print it to reassure that the right settings are being used.
-# Don't use python3 because it doesn't work in git bash on Windows.
-command=$(python "$(dirname -- "$0")/autoninja.py" "$@")
+command=$($python3 "$depot_tools/autoninja.py" "$@")
 if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
   echo "$command"
 fi
 if eval "$command"; then
   if [ "$NINJA_SUMMARIZE_BUILD" == "1" ]; then
-    python3 "$(dirname -- "$0")/post_build_ninja_summary.py" "$@"
+    $python3 "$depot_tools/post_build_ninja_summary.py" "$@"
   fi
 
   # Collect ninjalog from googler.
-  python3 "$(dirname -- "$0")/ninjalog_uploader_wrapper.py" --cmd $command
+  $python3 "$depot_tools/ninjalog_uploader_wrapper.py" --cmd $command
   exit
 fi
 
 # Collect ninjalog from googler.
-python3 "$(dirname -- "$0")/ninjalog_uploader_wrapper.py" --cmd $command
+$python3 "$depot_tools/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.