Added default value for the quiet parameter

The activate_end method is used in infra/bootstrap/build_deps.py, which doesn't
pass the value for this parameter.

R=pgervais@chromium.org
BUG=503067

Review URL: https://codereview.chromium.org/1213113002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@295893 0039d316-1c4b-4281-b951-d872f2087c98

bootstrap/bootstrap.py

@@ -154,7 +154,7 @@ def install(deps):
          os.path.join(ROOT, '.wheelcache'), '-f', ipath] + requirements)
 
 
-def activate_env(env, deps, quiet):
+def activate_env(env, deps, quiet=False):
   if hasattr(sys, 'real_prefix'):
     LOGGER.error('Already activated environment!')
     return