extensions: fix dev controls spacing issue.

chrome/browser/resources/extensions/extensions.css

 /* Copyright (c) 2012 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 html.loading * {
   transition-duration: 0 !important;
 }
 
 html:not(.focus-outline-visible)
 :enabled:focus:-webkit-any(input[type='checkbox'], input[type='radio']) {
@@ skipping 23 matching lines @@
 
 .dev-mode #dev-controls {
   border-bottom: 1px solid #eee;
 }
 
 #dev-controls > * {
   padding: 8px 3px;
 }
 
 #dev-controls .button-container {
   display: flex;

[Dan Beam, 2015/02/04 20:55:02]

this collapses whitespace.  all other webui buttons aren't yet using flex, and
have whitespace inbetween them.  5px is an approximation that looks good on all
platforms/fonts (I originally added a   in the middle but didn't like
that).

[not at google - send to devlin, 2015/02/04 21:01:53]

I suppose even clearer would be to make the selector below

#dev-controls .button-container button:not(:last-of-type)

if it's the (#dev-controls .button-container) selector which is putting this in
the state.

concretely, in case you have like

<div id=dev-controls>
  <button>hi</button>
  <div id=button-container>
    <button>....
  </div>
</div>

which wouldn't make much sense.

[Dan Beam, 2015/02/04 21:04:31]

Done.

   flex-wrap: wrap;
 }
 
 #dev-controls button {
   white-space: nowrap;
 }
 
+#dev-controls button:not(:last-of-type) {
+  -webkit-margin-end: 5px;

[not at google - send to devlin, 2015/02/04 20:39:01]

I see that other WebUI buttons look fine, and don't need special treatment (or
at least they already seem to have it). Why is this code needed?

+}
+
 #apps-developer-tools-promo {
   align-items: center;
   border-top: 1px solid #eee;
   display: flex;
   font-size: 13px;
 }
 
 #apps-developer-tools-promo img {
   -webkit-margin-end: 5px;
   -webkit-margin-start: -5px;
@@ skipping 344 matching lines @@
 /* Sideload Wipeout */
 
 .sideload-wipeout-learn-more {
   text-decoration: none;
 }
 
 .sideload-wipeout-banner .page-banner-text {
   -webkit-padding-start: 8px;
   background-image: none;
 }