[CSS Grid Layout] Add test coverage for Implicit Named Areas

Specs state in section 5.2.2 that explicitly adding named lines of the form
<foo-start>/<foo-end> should effectively create a named grid area (named foo
in this case). We got this for free in r174021 as a nice side effect of
implementing the resolution for named grid lines. We just lack proper test
coverage to ensure that we don't regress.

Review URL: https://codereview.chromium.org/313263003

git-svn-id: svn://svn.chromium.org/blink/trunk@176300 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/fast/css-grid-layout/named-grid-lines-with-named-grid-areas-resolution-expected.txt

@@ -21,3 +21,4 @@ PASS
 PASS
 PASS
 PASS
+PASS

third_party/WebKit/LayoutTests/fast/css-grid-layout/named-grid-lines-with-named-grid-areas-resolution.html

@@ -46,6 +46,11 @@
       grid-template-rows: 50px (c-end) 100px (c-end) 200px (c-end);
   }
 
+  .gridImplicitArea {
+      grid-template-columns: 50px (z-start) 100px (z-end) 200px;
+      grid-template-rows: 50px (z-start) 100px 200px (z-end);
+  }
+
 </style>
 <script src="../../resources/check-layout.js"></script>
 </head>
@@ -239,5 +244,14 @@
   </div>
 </div>
 
+<!-- Check that <custom-ident>-{start|end} named lines define implicit named areas -->
+<div style="position: relative">
+  <div class="grid gridAreas gridImplicitArea">
+    <div class="sizedToGridArea" style="grid-column: z;" data-offset-x="50" data-offset-y="0" data-expected-width="100" data-expected-height="50"></div>
+    <div class="sizedToGridArea" style="grid-row: z;" data-offset-x="0" data-offset-y="50" data-expected-width="50" data-expected-height="300"></div>
+    <div class="sizedToGridArea" style="grid-column-end: z; grid-row-start: z;" data-offset-x="50" data-offset-y="50" data-expected-width="100" data-expected-height="100"></div>
+  </div>
+</div>
+
 </body>
 </html>