Subpixel precision for clientWidth, offsetWidth, scrollTop et al

Change clientHeight, clientWidth, clientLeft, clientTop, scrollTop,
scrollLeft, scrollWidth, scrollHeight, offsetWidth, offsetHeight,
offsetTop, and offsetLeft to return precise floating point values
instead of rounded int values.

This matches the latest draft of the CSS OM Specification [1] and the
opt-in IE10 behavior [2].

Feature dashboard entry:
  http://www.chromestatus.com/features/5497402177880064

Intent to Implement & Ship:
  https://groups.google.com/a/chromium.org/d/msg/blink-dev/_Q7A4AQBFKY/S4ahQ5iE28QJ

R=ch.dumez@samsung.com, esprehn@chromium.org, leviw@chromium.org
BUG=360889
TEST=fast/sub-pixel/cssom-subpixel-precision.html,
     fast/sub-pixel/scroll-offset-precision.html

1: http://www.w3.org/TR/cssom-view/
2: http://blogs.msdn.com/b/ie/archive/2012/02/17/sub-pixel-rendering-and-the-css-object-model.aspx

Review URL: https://codereview.chromium.org/225263021

git-svn-id: svn://svn.chromium.org/blink/trunk@173053 bbb929c8-8fbe-4397-9dbb-9b2b20218538