Bug 1444301 - Update close icon to use Photon icon; r=jryans

This is the icon taken from https://design.firefox.com/icons/viewer/#close

MozReview-Commit-ID: B3BdbJf1W1f

--HG--
extra : rebase_source : a55c4d1e96ea96b8fec90c31674ac486eb6eed92

devtools/client/themes/images/close.svg

@@ -1,6 +1,6 @@
 <!-- This Source Code Form is subject to the terms of the Mozilla Public
    - License, v. 2.0. If a copy of the MPL was not distributed with this
    - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
-<svg width="16" height="16" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg" fill="context-fill #0b0b0b">
-  <path d="M8.707 8l4.23 4.23a.5.5 0 1 1-.707.707L8 8.707l-4.23 4.23a.5.5 0 1 1-.707-.707L7.293 8l-4.23-4.23a.5.5 0 1 1 .707-.707L8 7.293l4.23-4.23a.5.5 0 0 1 .707.707L8.707 8z" fill-rule="evenodd"/>
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16">
+  <path fill="context-fill" d="M9.061 8l3.47-3.47a.75.75 0 0 0-1.061-1.06L8 6.939 4.53 3.47a.75.75 0 1 0-1.06 1.06L6.939 8 3.47 11.47a.75.75 0 1 0 1.06 1.06L8 9.061l3.47 3.47a.75.75 0 0 0 1.06-1.061z"></path>
 </svg>

devtools/client/themes/toolbox.css

@@ -159,8 +159,8 @@
 
 /* Toolbox controls */
 
-
 #toolbox-close::before {
+  fill: var(--theme-toolbar-photon-icon-color);
   background-image: var(--close-button-image);
 }