Don't paint line-clamped lines.

This matches the proposal in
w3c/csswg-drafts#10816, and creates much better
behavior.

Differential Revision: https://phabricator.services.mozilla.com/D157578

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1791226
gecko-commit: cf9f54eef33465d089f850a55da5734a911fdac4
gecko-reviewers: layout-reviewers, dshin

css/css-overflow/line-clamp/reference/webkit-line-clamp-040-ref.html

@@ -11,6 +11,4 @@
 </style>
 <div class="clamp">Line 1
 Line 2…
-Line 3
-Line 4
-Line 5</div>
+</div>

css/css-overflow/line-clamp/webkit-line-clamp-050.html

@@ -16,15 +16,11 @@
     border: medium solid green;
     padding: 15px;
   }
-  span {
-    /* TODO: Remove once we don't paint clamped lines */
-    color: transparent;
-  }
 </style>
 <div class="clamp">
   Line1
-  <div>Line2<br><span>Line3</span></div>
-  <span>Line4</span>
+  <div>Line2<br>Line3</div>
+  Line4
   <div>Line5<br>Line6</div>
   Line7
 </div>