Use double in translate and perspective transforms instead of Length where appropriate

This patch changes the z component of translate and the perspective value
from Length to double. The Length class usually means we allow at least
<length> and <percent>, while values that are only <length>s usually get
stored as floats or doubles.

R=dstockwell
BUG=

Review URL: https://codereview.chromium.org/114313004

git-svn-id: svn://svn.chromium.org/blink/trunk@164218 bbb929c8-8fbe-4397-9dbb-9b2b20218538