depot_tools: Execute commands using Python 2 on Windows when GCLIENT_PY3=0.

Change-Id: Ib51acc5671c7b60219f64bcb73a70f85a51779a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2140817Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

fetch.bat

@@ -13,7 +13,12 @@ set PATH=%PATH%;%~dp0
 
 :: Defer control.
 IF "%GCLIENT_PY3%" == "1" (
-  vpython3 "%~dp0\fetch.py" %*
+  :: Explicitly run on Python 3
+  call vpython3 "%~dp0\fetch.py" %*
+) ELSE IF "%GCLIENT_PY3%" == "0" (
+  :: Explicitly run on Python 2
+  call vpython "%~dp0\fetch.py" %*
 ) ELSE (
-  vpython "%~dp0\fetch.py" %*
+  :: Run on Python 2, allows default to be flipped.
+  call vpython "%~dp0\fetch.py" %*
 )

gclient.bat

@@ -17,8 +17,12 @@ set PATH=%PATH%;%~dp0
 
 :: Defer control.
 IF "%GCLIENT_PY3%" == "1" (
-  :: TODO(1003139): Use vpython3 once vpython3 works on Windows.
-  call python3 "%~dp0gclient.py" %*
+  :: Explicitly run on Python 3
+  call vpython3 "%~dp0gclient.py" %*
+) ELSE IF "%GCLIENT_PY3%" == "0" (
+  :: Explicitly run on Python 2
+  call vpython "%~dp0gclient.py" %*
 ) ELSE (
-  call python "%~dp0gclient.py" %*
+  :: Run on Python 2, allows default to be flipped.
+  call vpython "%~dp0gclient.py" %*
 )

roll-dep.bat

@@ -9,4 +9,13 @@ setlocal
 set PATH=%PATH%;%~dp0
 
 :: Defer control.
-python "%~dp0\roll_dep.py" %*
+IF "%GCLIENT_PY3%" == "1" (
+  :: Explicitly run on Python 3
+  call vpython3 "%~dp0\roll_dep.py" %*
+) ELSE IF "%GCLIENT_PY3%" == "0" (
+  :: Explicitly run on Python 2
+  call vpython "%~dp0\roll_dep.py" %*
+) ELSE (
+  :: Run on Python 2, allows default to be flipped.
+  call vpython "%~dp0\roll_dep.py" %*
+)