DevTools: Add support for adding folders to workspace with drag and drop (renderer part)

Source/devtools/front_end/sourcesView.css

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 12 matching lines @@
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #sources-panel-sources-view {
     flex: auto;
+    position: relative;
 }
 
 #sources-editor-container-tabbed-pane {
     flex: auto;
 }
 
 #sources-editor-container-tabbed-pane .tabbed-pane-header-tab {
     border-bottom: 1px solid #BBB;
 }
 
@@ skipping 12 matching lines @@
     -webkit-user-select: text;
 }
 
 #sources-panel-sources-view .sources-toggle-pretty-print-status-bar-item .glyph {
     -webkit-mask-position: -256px -24px;
 }
 
 #sources-panel-sources-view .sources-toggle-pretty-print-status-bar-item.toggled .glyph {
     background-color: rgb(66, 129, 235);
 }
+
+#sources-panel-sources-view .drag-mask
+    background-color: transparent;
+    z-index: 1000;
+}