[LayoutNG] Don't allow line height quirk inside list items.

This makes us match legacy engine behavior more closely, and both Edge
and Firefox have something similar, although Firefox triggers based on
element type (LI), and not display type (list-item). There's currently
nothing in the spec [1] that suggests such special behavior for list
items.

[1] https://quirks.spec.whatwg.org/#the-line-height-calculation-quirk

This fix makes the line height differences go away in the test
tables/mozilla/bugs/bug23235.html (but note that it will still require
a rebaseline, because of minor text rendering differences between
legacy and NG).

See whatwg/quirks#38 for spec discussion.

Change-Id: I396c0c85454feeaa45a0a5953b9e6b7c475d131e

quirks/line-height-in-list-item.tentative.html

@@ -0,0 +1,12 @@
+<!-- quirks mode -->
+<link rel="author" title="Morten Stenshorne" href="[email protected]">
+<link rel="help" href="https://quirks.spec.whatwg.org/#the-line-height-calculation-quirk">
+<link rel="match" href="reference/green-100px-square-no-red.html">
+<meta name="assert" content="List items force strict line height; see github issue <ENTER GITHUB ISSUE HERE>">
+<p>There should be a filled green square below, and no red.</p>
+<div style="width:100px; height:100px; background:red;">
+  <li style="line-height:50px; color:green; background:green;">
+    <br>
+    <div style="display:inline-block; width:1px; height:1px;"></div>
+  </li>
+</div>

quirks/reference/green-100px-square-no-red.html

@@ -0,0 +1,4 @@
+<!-- quirks mode -->
+<link rel="author" title="Morten Stenshorne" href="[email protected]">
+<p>There should be a filled green square below, and no red.</p>
+<div style="width:100px; height:100px; background:green;"></div>