Fix scrollWidth/Height to not truncate scroll offset

Fix bug in Element::scrollWidth/scrollHeight where the scroll offset
was truncated to an integer value before being adjusted for zoom.

TEST=fast/sub-pixel/clientwidth-matches-scrollwidth-for-float.html
BUG=381039

Review URL: https://codereview.chromium.org/319633004

git-svn-id: svn://svn.chromium.org/blink/trunk@175698 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/dom/Element.cpp

@@ -804,7 +804,7 @@ double Element::scrollWidth()
 {
     document().updateLayoutIgnorePendingStylesheets();
     if (RenderBox* rend = renderBox())
-        return adjustForAbsoluteZoom(rend->scrollWidth(), rend);
+        return adjustLayoutUnitForAbsoluteZoom(rend->scrollWidth(), *rend).toDouble();
     return 0;
 }
 
@@ -812,7 +812,7 @@ double Element::scrollHeight()
 {
     document().updateLayoutIgnorePendingStylesheets();
     if (RenderBox* rend = renderBox())
-        return adjustForAbsoluteZoom(rend->scrollHeight(), rend);
+        return adjustLayoutUnitForAbsoluteZoom(rend->scrollHeight(), *rend).toDouble();
     return 0;
 }