DevTools: add console viewport visibility test cleanup

Besides minor cleanup, in this CL:
- Viewport selection.js and stick-to-bottom.js tests no longer
  do "verifyViewportIsTallEnough". This sort of test is now covered
  by "console-viewport-indices.js"
- The actual visible indices can be determined in tests with a
  helper that uses getBoundingClientRect()

Bug: 803537
Change-Id: I7642051b6a5eb358c536ae29568ec9de91a02a8b
Reviewed-on: https://chromium-review.googlesource.com/874778
Reviewed-by: Joel Einbinder <[email protected]>
Reviewed-by: Andrey Lushnikov <[email protected]>
Commit-Queue: Erik Luo <[email protected]>
Cr-Commit-Position: refs/heads/master@{#531158}

front_end/console_test_runner/ConsoleTestRunner.js

@@ -576,3 +576,32 @@ ConsoleTestRunner.dumpStackTraces = function() {
     }
   }
 };
+
+/**
+ * Returns actual visible indices. Messages in the margin are treated as NOT visible.
+ * @return {!{first: number, last: number, count: number}}
+ */
+ConsoleTestRunner.visibleIndices = function() {
+  var consoleView = Console.ConsoleView.instance();
+  var viewport = consoleView._viewport;
+  var viewportRect = viewport.element.getBoundingClientRect();
+  var viewportPadding = parseFloat(window.getComputedStyle(viewport.element).paddingTop);
+  var first = -1;
+  var last = -1;
+  var count = 0;
+  for (var i = 0; i < consoleView._visibleViewMessages.length; i++) {
+    // Created message elements may have a bounding rect, but not be connected to DOM.
+    var item = consoleView._visibleViewMessages[i];
+    if (!item._element || !item._element.isConnected)
+      continue;
+    var itemRect = item._element.getBoundingClientRect();
+    var isVisible = (itemRect.bottom > viewportRect.top + viewportPadding + 1) &&
+        (itemRect.top <= viewportRect.bottom - viewportPadding - 1);
+    if (isVisible) {
+      first = first === -1 ? i : first;
+      last = i;
+      count++;
+    }
+  }
+  return {first, last, count};
+};