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

chrome/browser/resources/extensions/extensions.css

Index: chrome/browser/resources/extensions/extensions.css
diff --git a/chrome/browser/resources/extensions/extensions.css b/chrome/browser/resources/extensions/extensions.css
index ee9ec5297f96264dd85964d09ad0b446913a9748..87e6d0b905c8c3661b50b3d92d13ff6452e420db 100644
--- a/chrome/browser/resources/extensions/extensions.css
+++ b/chrome/browser/resources/extensions/extensions.css
@@ -89,6 +89,28 @@ html.loading * {
   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;