[vpython] Implement developer bypass parser in vpython wrapper.

This will allow users on unsupported platforms to successfully bypass vpython
via the recognized environment variable.

R=tandrii@chromium.org, vadimsh@chromium.org

Change-Id: Id63019443321d9f63550fc11e7a4ae3f684b17e7
Reviewed-on: https://chromium-review.googlesource.com/927695
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

vpython

 #!/bin/bash
 
-# Copyright 2017 The Chromium Authors. All rights reserved.
+# Copyright 2018 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+if [[ $VPYTHON_BYPASS == "manually managed python not supported by chrome operations" ]]
+then
+  NEWARGS=()
+  while [[ $# -gt 0 ]]
+  do
+    case "$1" in
+      -vpython-tool*) # these tools all do something vpython related and quit
+        exit 0
+        ;;
+      -vpython*=*) # delete any vpython-specific flag (w/ attached argument)
+        shift
+        ;;
+      -vpython*) # delete any vpython-specific flag (w/ separate argument)
+        shift
+        shift
+        ;;
+      --)     # stop parsing
+        NEWARGS+=( "$@" )
+        break
+        ;;
+      *)      # regular arg
+        NEWARGS+=( "$1" )
+        shift
+        ;;
+    esac
+  done
+  exec "python" "${NEWARGS[@]}"
+fi
+
+
 MYPATH=$(dirname "${BASH_SOURCE[0]}")
 
 source "$MYPATH/cipd_bin_setup.sh"