Remove z-index of OneGoogleBar to place it above ntp-contents.

The z-index fix for the theme attribution placed the OneGoogleBar behind the ntp-contents div, which made the OneGoogleBar unclickable. Removing the z-index of OneGoogleBar fixes it from being rendered below ntp-contents.

Bug: 827969
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I5d36f8b1c727a614ef6e2707a440af5413ecbc57
Reviewed-on: https://chromium-review.googlesource.com/990235
Commit-Queue: Kristi Park <kristipark@chromium.org>
Reviewed-by: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547616}

chrome/browser/resources/local_ntp/local_ntp.css

@@ -530,7 +530,6 @@ html[dir=rtl] #attribution,
   right: 0;
   top: 0;
   transition: opacity 130ms;
-  z-index: 1;
 }
 
 #one-google.hidden {