Web Animations: Force opacity to range [0, 1)

This forces a layer to be created, as we create a layer for any opacity value
other than 1.

BUG=322272
R=shans

Review URL: https://codereview.chromium.org/100833003

git-svn-id: svn://svn.chromium.org/blink/trunk@163197 bbb929c8-8fbe-4397-9dbb-9b2b20218538