DevTools: Make sure a horizontal scrollbar doesn't appear in SuggestBox

BUG=none

Review-Url: https://codereview.chromium.org/2607103004
Cr-Commit-Position: refs/heads/master@{#441779}

third_party/WebKit/Source/devtools/front_end/ui/ListControl.js

@@ -54,7 +54,7 @@ UI.ListControl = class {
    */
   constructor(delegate, mode) {
     this.element = createElement('div');
-    this.element.style.overflow = 'auto';
+    this.element.style.overflowY = 'auto';
     this._topElement = this.element.createChild('div');
     this._bottomElement = this.element.createChild('div');
     this._firstIndex = 0;

third_party/WebKit/Source/devtools/front_end/ui/suggestBox.css

@@ -70,6 +70,7 @@
     box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.05),
                 0 2px 4px rgba(0, 0, 0, 0.2),
                 0 2px 6px rgba(0, 0, 0, 0.1);
+    overflow-x: hidden;
 }
 
 .suggest-box .suggest-box-content-item {