DevTools: Prevent horizontal scrolling in computed style sidebar

Bug: 829244
Change-Id: Idc426e6f0963a30340569c2d2835266d325f290e
Reviewed-on: https://chromium-review.googlesource.com/1079719Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567866}

third_party/blink/renderer/devtools/front_end/elements/elementsPanel.css

@@ -51,7 +51,7 @@
 }
 
 .style-panes-wrapper {
-    overflow: auto;
+    overflow: hidden auto;
 }
 
 .style-panes-wrapper > div:not(:first-child) {