Computed style for font-size should not be rounded to integer

As the crbug.com/290411 was implemented, computed style for font-size
should return floating value.

BUG=397117

Review URL: https://codereview.chromium.org/607293002

git-svn-id: svn://svn.chromium.org/blink/trunk@182931 bbb929c8-8fbe-4397-9dbb-9b2b20218538