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

Source/devtools/front_end/inspectorStyle.css

Index: Source/devtools/front_end/inspectorStyle.css
diff --git a/Source/devtools/front_end/inspectorStyle.css b/Source/devtools/front_end/inspectorStyle.css
index f342f52dbcf356461783031ec6f9223a7cde2ea1..8fe6ff380d7c169f70c0f5d2b1c3f8bb2b22bc80 100644
--- a/Source/devtools/front_end/inspectorStyle.css
+++ b/Source/devtools/front_end/inspectorStyle.css
@@ -128,7 +128,7 @@ body.undocked .inspector-view-close-button {
 }
 
 .panel {
-    display: none !important;
+    display: flex;
     overflow: hidden;
     position: absolute;
     top: 0;
@@ -138,10 +138,6 @@ body.undocked .inspector-view-close-button {
     z-index: 0;
 }
 
-.panel.visible {
-    display: flex !important;
-}
-
 iframe.extension {
     flex: auto;
     width: 100%;