Use doubles for keyframe offsets throughout the animation stack for improved precision

The CSS Animations spec does not specify whether offsets should be handled as
floats or doubles.

Floats provide 24 bits of precision and the range of these offsets is [0, 100].
This means that it's possible to see rendering errors in animations where the
duration exceeds 2^24 / 100 * 1/60 = 2796s = 47mins.

Doubles provide exact rendering for animations up to 95,140 years in duration.

R=shans

Review URL: https://chromiumcodereview.appspot.com/22389003

git-svn-id: svn://svn.chromium.org/blink/trunk@156065 bbb929c8-8fbe-4397-9dbb-9b2b20218538