ChromeOS: make login header bar background transparent.

Bug: 736904
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I179d50c4dd3e2c835f06145e2c453c0012f1e001
Reviewed-on: https://chromium-review.googlesource.com/578952Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488377}

chrome/browser/resources/chromeos/login/header_bar.css

@@ -8,15 +8,11 @@
   left: 0;
   min-height: 34px;  /* Should be consistent with .header-bar-item's height. */
   padding-bottom: 6px;
+  padding-top: 7px;
   position: absolute;
   right: 0;
 }
 
-#login-header-bar {
-  background-color: rgba(0, 0, 0, 0.6);
-  padding-top: 7px;
-}
-
 #login-header-bar.full-header-background {
   background-color: #000;
 }