[DevTools] New harness for inspector-protocol layout tests

third_party/WebKit/LayoutTests/inspector-protocol/page/get-layout-metrics.js

+(async function(testRunner) {
+  let {page, session, dp} = await testRunner.startHTML(`
+    <style>
+    body {
+        min-height: 2000px;
+    }
+    </style>
+  `, 'Tests that Page.getLayoutMetrics returns reasonable values.');
+
+  session.evaluate('window.scrollTo(0, 200);');
+  var message = await dp.Page.getLayoutMetrics();
+  if (message.error) {
+    testRunner.log(message.error.message);
+    testRunner.completeTest();
+    return;
+  }
+
+  testRunner.logObject(message.result.layoutViewport, 'LayoutViewport: ');
+  testRunner.logObject(message.result.visualViewport, 'VisualViewport: ');
+  testRunner.completeTest();
+})