Expose fractional scroll offsets to JS

We've tried in the past to expose fractional scroll offsets and
positioning to the web, but hit web compatibility problems (see http://crbug.com/360889).

We'd like to try again to expose just fractional scroll OFFSETS
(eg. scrollLeft/scrollTop) without changing the higher-risk
position/measurement APIs (offsetTop, scrollHeight, etc.).

This CL exposes the fractions we have today in blink (for the
special case of browser zoom) to the scroll offset properties
and scrolling methods.  This will become much more usefull (eg.
for high-dpi) when http://crbug.com/414283 is complete.

Intent to ship: https://groups.google.com/a/chromium.org/forum/#!searchin/blink-dev/subpixel$20scrollTop/blink-dev/_Q7A4AQBFKY/S4ahQ5iE28QJ

BUG=373731

Review URL: https://codereview.chromium.org/563983004

git-svn-id: svn://svn.chromium.org/blink/trunk@182423 bbb929c8-8fbe-4397-9dbb-9b2b20218538