DevTools: Fix UI glitch with vertical scroll bar in settings dialog.

Source/devtools/front_end/helpScreen.css

 .help-window-outer {
     position: absolute !important;
     top: 0;
     left: 0;
     right: 0;
     bottom: 0;
     z-index: 2000;
 }
 
 .help-window-main {
@@ skipping 563 matching lines @@
     flex: 0 0 auto;
     color: #333;
     font-size: 14px;
     margin: 0;
     padding: 14px 17px 14px;
 }
 
 .settings-dialog .contents {
     flex: 1 1 auto;
     padding: 0 17px;
-    overflow: auto;
+    overflow-x: hidden;
+    overflow-y: auto;
 }
 
 .settings-dialog .block-header {
     color: black;
     font-size: 1.2em;
     margin-bottom: 0.8em;
 }
 
 .blackbox-dialog .columns-header {
     color: black;
@@ skipping 38 matching lines @@
     display: none;
 }
 
 .settings-experiment-hidden label {
     background-color: #ddd;
 }
 
 .settings-developer-mode .settings-experiment-hidden {
     display: block;
 }