AnimationTimeline::serviceAnimations should reserveInitialCapacity

Rather than reallocating the players vector, we can just reserve exactly the
correct capacity ahead of time. I didn't see these mallocs on a profile, but
this code is executed every frame during composited animations.

R=dstockwell@chromium.org

Review URL: https://codereview.chromium.org/353083002

git-svn-id: svn://svn.chromium.org/blink/trunk@177053 bbb929c8-8fbe-4397-9dbb-9b2b20218538