Fix test tolerance for fixed-point line layout.

LayoutTests/svg/text/text-rect-precision.html

Index: LayoutTests/svg/text/text-rect-precision.html
diff --git a/LayoutTests/svg/text/text-rect-precision.html b/LayoutTests/svg/text/text-rect-precision.html
index b7587cd46b15cdfbedb58379290b59580c7e6b20..f05043d0d2082748b24a5799c688aab6eed4b2d8 100644
--- a/LayoutTests/svg/text/text-rect-precision.html
+++ b/LayoutTests/svg/text/text-rect-precision.html
@@ -15,6 +15,15 @@
     
     var hasSubpixelPrecision = false;
     
+    function subpixelTolerance(testElement)
+    {
+        // Due to fixed-point rounding, each single-character measurement may differ by up to
+        // one LayoutUnit (i.e., 0.16 pixel) from the same character's measurement in the full
+        // string.
+        var str = testElement.firstChild.nodeValue;
+        return str.length * 0.16;
+    }
+
     function measureText(testElement)
     {
         var measureElement = document.getElementById('measure');
@@ -40,7 +49,7 @@
     var el = document.getElementById('test');
     var elementWidth = el.getBoundingClientRect().width;
     var textWidth = measureText(el);
-    var tolerance = hasSubpixelPrecision ? 0.1 : 2; // enclosing may expand up to one pixel in each direction.
+    var tolerance = hasSubpixelPrecision ? subpixelTolerance(el) : 2; // enclosing may expand up to one pixel in each direction.
     if (Math.abs(elementWidth - textWidth) <= tolerance)
         testPassed('Width of text element is the sum of the width of all characters.');
     else