roll-dep: Make it possible to run using Python 3.

Run roll-dep using Python 3 when GCLIENT_PY3 is set to 1.

Bug: 984182
Change-Id: I511b4cf34b4f5522218a24dff6d0ce9ef5c36c4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2080614
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

roll-dep

@@ -5,4 +5,17 @@
 
 base_dir=$(dirname "$0")
 
-PYTHONDONTWRITEBYTECODE=1 exec python "$base_dir/roll_dep.py" "$@"
+# Ensure that "depot_tools" is somewhere in PATH so this tool can be used
+# standalone, but allow other PATH manipulations to take priority.
+PATH=$PATH:$base_dir
+
+if [[ $GCLIENT_PY3 = 1 ]]; then
+  # Explicitly run on Python 3
+  PYTHONDONTWRITEBYTECODE=1 exec vpython3 "$base_dir/roll_dep.py" "$@"
+elif [[ $GCLIENT_PY3 = 0 ]]; then
+  # Explicitly run on Python 2
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/roll_dep.py" "$@"
+else
+  # Run on Python 2 for now, allows default to be flipped.
+  PYTHONDONTWRITEBYTECODE=1 exec vpython "$base_dir/roll_dep.py" "$@"
+fi