Add 'left: 0px' to transition tests that transition 'left'

In the existing transitions implementation, transitioning from auto (the
default if 'left' is unspecified) to some pixel amount will snap to 0px
then transition from there. This patch explicitly sets 'left: 0px' in
tests which transition the 'left' property so they do not depend on this
old behaviour (which the Web Animations implementation doesn't do). We
also set 'top: 0px' in one test where it's used.

BUG=303430

Review URL: https://codereview.chromium.org/36173003

git-svn-id: svn://svn.chromium.org/blink/trunk@160295 bbb929c8-8fbe-4397-9dbb-9b2b20218538