[DevTools] Place modal elements (suggest box, popover) above splitters.

BUG=353090

Review URL: https://codereview.chromium.org/203683005

git-svn-id: svn://svn.chromium.org/blink/trunk@169745 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/devtools/front_end/popover.css

@@ -2,7 +2,7 @@
     position: absolute;
     -webkit-border-image: url(Images/popoverBackground.png) 25 25 25 25;
     border-width: 25px;
-    z-index: 100;
+    z-index: 600;
     pointer-events: none;
 }
 

third_party/WebKit/Source/devtools/front_end/textPrompt.css

@@ -34,7 +34,7 @@
     display: block;
     border: 1px solid black;
     padding: 2px;
-    z-index: 100;
+    z-index: 600;
 }
 
 .suggest-box .container {