Fix console viewport test failures with scroll anchoring.

Update defaultConsoleRowHeight to account for the 1px border, and grow a visible
item instead of an offscreen one when testing setStickToBottom.

See bug for detailed analysis.

BUG=624534

Review-Url: https://codereview.chromium.org/2317343006
Cr-Commit-Position: refs/heads/master@{#418681}

third_party/WebKit/LayoutTests/inspector/console/console-viewport-stick-to-bottom.html

@@ -88,13 +88,13 @@ function test()
 
         function testViewportMutationsShouldPreserveStickToBottom(next)
         {
-            viewport._contentElement.children[1].innerText = "More than 2 lines: foo\n\nbar";
+            viewport._contentElement.lastChild.innerText = "More than 2 lines: foo\n\nbar";
             dumpAndContinue(onMessagesDumped);
 
             function onMessagesDumped()
             {
                 viewport.setStickToBottom(false);
-                viewport._contentElement.children[1].innerText = "More than 3 lines: foo\n\n\nbar";
+                viewport._contentElement.lastChild.innerText = "More than 3 lines: foo\n\n\nbar";
                 dumpAndContinue(next);
             }
         },

third_party/WebKit/Source/devtools/front_end/console/ConsoleViewMessage.js

@@ -118,7 +118,9 @@ WebInspector.ConsoleViewMessage.prototype = {
     {
         if (this._cachedHeight)
             return this._cachedHeight;
-        const defaultConsoleRowHeight = 18;  // Sync with consoleView.css
+        // This value reflects the 18px min-height of .console-message, plus the
+        // 1px border of .console-message-wrapper. Keep in sync with consoleView.css.
+        const defaultConsoleRowHeight = 19;
         if (this._message.type === WebInspector.ConsoleMessage.MessageType.Table) {
             var table = this._message.parameters[0];
             if (table && table.preview)