Compute getComputedStyle(x).lineHeight accurately while zooming.

Use accurated and not rounded off values for computing font size
and for line-height calculations use float, instead of integer.

BUG=277378

Review URL: https://chromiumcodereview.appspot.com/25024002

git-svn-id: svn://svn.chromium.org/blink/trunk@158509 bbb929c8-8fbe-4397-9dbb-9b2b20218538