Include a mark as read X under the scroll to unread button

res/css/views/rooms/_TopUnreadMessagesBar.scss

@@ -56,3 +56,17 @@ limitations under the License.
     mask-position: 9px 13px;
     background: $roomtile-name-color;
 }
+
+.mx_TopUnreadMessagesBar_markAsRead {
+    display: block;
+    width: 18px;
+    height: 18px;
+    background-image: url('$(res)/img/cancel.svg');
+    background-position: center;
+    background-size: 10px;
+    background-repeat: no-repeat;
+    background-color: $primary-bg-color;
+    border: 1.3px solid $roomtile-name-color;
+    border-radius: 99px;
+    margin: 5px auto;
+}

src/components/views/rooms/TopUnreadMessagesBar.js

@@ -27,6 +27,7 @@ export default createReactClass({
 
     propTypes: {
         onScrollUpClick: PropTypes.func,
+        onCloseClick: PropTypes.func,
     },
 
     render: function() {
@@ -36,6 +37,10 @@ export default createReactClass({
                     title={_t('Jump to first unread message.')}
                     onClick={this.props.onScrollUpClick}>
                 </AccessibleButton>
+                <AccessibleButton className="mx_TopUnreadMessagesBar_markAsRead"
+                    title={_t('Mark all as read')}
+                    onClick={this.props.onCloseClick}>
+                </AccessibleButton>
             </div>
         );
     },