Have autoninja.bat run depot_tools python3

This changes autoninja.bat so that it refers explicitly to the
depot_tools version of Python rather than depending on what is in the
path. This helps to make autoninja behave more consistently and makes it
easier for us to remove python.bat from the path.

Bug: 777069
Change-Id: I6d878f58e2f8be70d06a182d059caa9fbedaedbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2753458
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

autoninja.bat

@@ -8,7 +8,7 @@ setlocal
 set scriptdir=%~dp0
 
 REM Set unique build ID.
-FOR /f "usebackq tokens=*" %%a in (`python -c "import uuid; print(uuid.uuid4())"`) do set AUTONINJA_BUILD_ID=%%a
+FOR /f "usebackq tokens=*" %%a in (`%scriptdir%python-bin\python3.bat -c "import uuid; print(uuid.uuid4())"`) do set AUTONINJA_BUILD_ID=%%a
 
 REM If a build performance summary has been requested then also set NINJA_STATUS
 REM to trigger more verbose status updates. In particular this makes it possible
@@ -37,17 +37,17 @@ REM Don't use vpython - it is too slow to start.
 REM Don't use python3 because it doesn't work in git bash on Windows and we
 REM should be consistent between autoninja.bat and the autoninja script used by
 REM git bash.
-FOR /f "usebackq tokens=*" %%a in (`python %scriptdir%autoninja.py "%*"`) do echo %%a & %%a
+FOR /f "usebackq tokens=*" %%a in (`%scriptdir%python-bin\python3.bat %scriptdir%autoninja.py "%*"`) do echo %%a & %%a
 @if errorlevel 1 goto buildfailure
 
-REM Use call to invoke python script here, because we use python via python.bat.
-@if "%NINJA_SUMMARIZE_BUILD%" == "1" call python %scriptdir%post_build_ninja_summary.py %*
-@call python %scriptdir%ninjalog_uploader_wrapper.py --cmdline %*
+REM Use call to invoke python script here, because we use python via python3.bat.
+@if "%NINJA_SUMMARIZE_BUILD%" == "1" call %scriptdir%python-bin\python3.bat %scriptdir%post_build_ninja_summary.py %*
+@call %scriptdir%python-bin\python3.bat %scriptdir%ninjalog_uploader_wrapper.py --cmdline %*
 
 exit /b
 :buildfailure
 
-@call python %scriptdir%ninjalog_uploader_wrapper.py --cmdline %*
+@call %scriptdir%python-bin\python3.bat %scriptdir%ninjalog_uploader_wrapper.py --cmdline %*
 
 REM Return an error code of 1 so that if a developer types:
 REM "autoninja chrome && chrome" then chrome won't run if the build fails.