[LayoutNG] Add CalculateChildPercentageSize utility function.

Fixes percentage sized floats which were previously not using the
correct percentage resolution size.

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: Ie57909b7f452f5d59b8dfe2f899bedf253ab90ac
Reviewed-on: https://chromium-review.googlesource.com/1205200
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588959}

css/CSS2/normal-flow/float-percentage-resolution-quirks-mode.html

@@ -0,0 +1,10 @@
+<title>In quirks mode a float should resolve its percentage height against its first ancestor with a defined height.</title>
+<link rel="help" href="https://www.w3.org/TR/CSS22/visudet.html#Computing_widths_and_margins">
+<link rel="match" href="../../reference/ref-filled-green-100px-square-only.html">
+<p style="margin-top: 1em;">Test passes if there is a filled green square.</p>
+<div style="width:100px; height:100px; background:red;">
+  <div>
+    <div></div>
+    <div style="float:left; width:100%; height:100%; background:green;"></div>
+  </div>
+</div>