Kaleidoscope: Add slot into toolbar to put settings button

This CL adds a slot into the Kaleidoscope toolbar so that we can insert
a settings button.

Bug: b:150614426
Change-Id: Id417c9a9aed2f3c609283dd52262ab454f730b8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461940
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815749}

chrome/browser/resources/kaleidoscope/toolbar.html

@@ -8,4 +8,10 @@
     z-index: 99;
   }
 </style>
-<cr-toolbar id="toolbar" show-menu on-search-changed="onSearchChanged_" menu-label="[[menuLabel]]" page-name="[[pageName]]"></cr-toolbar>
+<cr-toolbar id="toolbar"
+            show-menu
+            on-search-changed="onSearchChanged_"
+            menu-label="[[menuLabel]]"
+            page-name="[[pageName]]">
+  <slot></slot>
+</cr-toolbar>