Guard the rework of documentElement|body.scrollTop/Left under a runtime flag

Blink is now (Nov 11, 2013) at a middle state of cross Web engine compatibility
with regards to documentElement and body scrollLeft/Top properties' behavior [1]:
1. documentElement works as per the spec.
2. body does not, and UseCounts the "improper" usage, keeping the existing
WebKit/Blink quirk behavior.

Given the amount of occurences that rely on the mutually exclusive behavior of
such properties [2], it is preferable to restore the original quirk behavior
and work on the new behavior under a runtime flag: ScrollTopLeftInterop.

In short, as Blink is now, websites that try to work around WebKit/Blink bug by
doing the below math are broken.

scrollOffset = documentElement.scrollTop + body.scrollTop [2]

[1] http://dev.w3.org/csswg/cssom-view/#dom-element-scrolltop
[2] http://bit.ly/17jiCMJ

BUG=314700
R=eseidel

Review URL: https://codereview.chromium.org/69143002

git-svn-id: svn://svn.chromium.org/blink/trunk@162166 bbb929c8-8fbe-4397-9dbb-9b2b20218538