Use pauseAnimation instead of suspendAnimations during layout tests

The suspendAnimations path involves a converting a time offset to a 
wall clock time, followed by a conversion to monotonic time in
GraphicsLayer::suspendAnimations. This not only introduces flake but
is also incorrect when cc/ is using high-res monotonic time on Windows.
The pauseAnimation path just sends a time offset to cc.

BUG=312060

Review URL: https://codereview.chromium.org/48953002

git-svn-id: svn://svn.chromium.org/blink/trunk@160861 bbb929c8-8fbe-4397-9dbb-9b2b20218538