LibWeb: Add undistributable space to GRID{MIN, MAX} instead of content

Adding undistributable space right before setting the content width is
incorrect when it's a percentage. Follow the specification and add it to
GRIDMIN and GRIDMAX instead.

Tests/LibWeb/Layout/input/table/border-spacing-with-percentage-width.html

@@ -0,0 +1,31 @@
+<style>
+    .left,
+    .right {
+        float: left;
+    }
+
+    .left {
+        width: 15%;
+    }
+
+    .middle {
+        float: left;
+        width: 61%;
+        border-spacing: 2px;
+    }
+
+    .right {
+        width: 24%
+    }
+</style>
+<div class="left">A</div>
+<table class="middle">
+    <tbody>
+        <tr>
+            <td>B</td>
+        </tr>
+    </tbody>
+</table>
+<div class="right">
+    C
+</div>