Add a first-class off-store install UI to chrome://extensions/.

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 * {
   -webkit-transition-duration: 0 !important;
 }
 
 /* Developer mode */
 
@@ skipping 71 matching lines @@
 .loading #get-more-extensions,
 #extension-settings-list:not(.empty-extension-list) ~ #no-extensions,
 .empty-extension-list ~ #get-more-extensions {
   display: none;
 }
 
 .extension-list-item-wrapper {
   margin: 23px 0;
 }
 
+#install-drop-target {
+  -webkit-transition: opacity 250ms linear;
+  background: #eee;
+  border: 1px dashed #ccc;
+  border-radius: 4px;
+  color: #777;
+  display: none;
+  font-weight: bold;
+  margin: 2em 2em 2em 0;
+  padding: 2em;
+  text-align: center;
+}
+
+body.drag-target #install-drop-target {
+  display: block;
+}
+
+#install-drop-target.active {
+  border-color: rgb(48, 57, 66);
+  color: rgb(48, 57, 66);
+}
+
 .extension-list-item {
   background-repeat: no-repeat;
   display: -webkit-box;
   min-height: 48px;
 }
 
 html[dir='rtl'] .extension-list-item {
   background-position: right;
 }
 
@@ skipping 119 matching lines @@
   right: auto;
 }
 
 .extension-list-item:not(:hover) .trash:not(:focus) {
   opacity: 0;
 }
 
 .extension-list-item-wrapper.may-not-disable .trash {
   visibility: hidden;
 }