Re-land: Added virtualenv for depot_tools

bootstrap/virtualenv/virtualenv_embedded/activate_this.py

+"""By using execfile(this_file, dict(__file__=this_file)) you will
+activate this virtualenv environment.
+
+This can be used when you must use an existing Python interpreter, not
+the virtualenv bin/python
+"""
+
+try:
+    __file__
+except NameError:
+    raise AssertionError(
+        "You must run this like execfile('path/to/activate_this.py', dict(__file__='path/to/activate_this.py'))")
+import sys
+import os
+
+old_os_path = os.environ.get('PATH', '')
+os.environ['PATH'] = os.path.dirname(os.path.abspath(__file__)) + os.pathsep + old_os_path
+base = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+if sys.platform == 'win32':
+    site_packages = os.path.join(base, 'Lib', 'site-packages')
+else:
+    site_packages = os.path.join(base, 'lib', 'python%s' % sys.version[:3], 'site-packages')
+prev_sys_path = list(sys.path)
+import site
+site.addsitedir(site_packages)
+sys.real_prefix = sys.prefix
+sys.prefix = base
+# Move the added items to the front of the path:
+new_sys_path = []
+for item in list(sys.path):
+    if item not in prev_sys_path:
+        new_sys_path.append(item)
+        sys.path.remove(item)
+sys.path[:0] = new_sys_path