DevTools: migrate sidebar pane's titleElement to use Toolbar.

Source/devtools/front_end/network/networkPanel.css

Index: Source/devtools/front_end/network/networkPanel.css
diff --git a/Source/devtools/front_end/network/networkPanel.css b/Source/devtools/front_end/network/networkPanel.css
index 3bfb07dc99664d3caacff9e7e02b9e559f84346b..bb000b46574f3f7cf103262902df29ee8f501b74 100644
--- a/Source/devtools/front_end/network/networkPanel.css
+++ b/Source/devtools/front_end/network/networkPanel.css
@@ -40,16 +40,12 @@
 }
 
 .network-item-view {
-    display: none;
-    background: white;
-}
-
-.network-item-view.visible {
     display: flex;
+    background: white;
 }
 
 .resource-timing-view {
-    display: none;
+    display: block;
     margin: 6px;
     color: rgb(30%, 30%, 30%);
 }
@@ -166,10 +162,6 @@
     margin: auto;
 }
 
-.resource-timing-view.visible {
-    display: block;
-}
-
 .resource-timing-view .network-timing-table {
     width: 100%;
 }