It's bad python practice to use a class-level member that is mutable. self.calls
is already initialized in __init__ so it's not useful (and dangerous) to have it
as a class member.

R=cmp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11801018

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@175368 0039d316-1c4b-4281-b951-d872f2087c98

Use it in subprocess2 and trial_dir to remove manually bookeeping.

R=dpranke@chromium.org
BUG=
TEST=


Review URL: http://codereview.chromium.org/8688011

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@111941 0039d316-1c4b-4281-b951-d872f2087c98