Allow Length::nonNanCalculatedValue to take a LayoutUnit

LayoutTests/fast/css/calc-rounding.html

Index: LayoutTests/fast/css/calc-rounding.html
diff --git a/LayoutTests/fast/css/calc-rounding.html b/LayoutTests/fast/css/calc-rounding.html
new file mode 100644
index 0000000000000000000000000000000000000000..0677d9f2a4bf5c664dc576ab91f6dd3fc143b35e
--- /dev/null
+++ b/LayoutTests/fast/css/calc-rounding.html
@@ -0,0 +1,25 @@
+<style>
+  #outerDiv, #innerDiv, #remainderDiv
+  {
+      background-color: green;
+      height: 100.5px;
+  }
+  #outerDiv
+  {
+      width: 100.5px;
+      background-color: red;
+  }
+  #innerDiv {
+      width: calc(100% - 10.5px);
+      float: left;
+  }
+  #remainderDiv
+  {
+      width: 10.5px;
+      float: left;
+  }
+</style>

[Julien - ping for review, 2015/01/26 10:55:22]

Let's add a description of what this is testing. The condition for passing
should also be added, unless it's converted to a check-layout.js file (see
below).

+<div id="outerDiv">
+  <div id="innerDiv"></div>
+  <div id="remainderDiv"></div>

[Julien - ping for review, 2015/01/26 10:55:22]

AFAICT this could be a check-layout.js test as what this is trying to do is
measure the size of the 2 inner divs. The upside is that this would be clearer
and faster.

+</div>