Devtools UI: Add high DPI chevron overflow icon

Source/devtools/front_end/elements/elementsTreeOutline.css

Index: Source/devtools/front_end/elements/elementsTreeOutline.css
diff --git a/Source/devtools/front_end/elements/elementsTreeOutline.css b/Source/devtools/front_end/elements/elementsTreeOutline.css
index 6ce620e7239b0fbad1d1ce04f8f8b58a75cc8673..45eeca07aa5bbd738ccc8f79e9bdfb9d517230cf 100644
--- a/Source/devtools/front_end/elements/elementsTreeOutline.css
+++ b/Source/devtools/front_end/elements/elementsTreeOutline.css
@@ -359,6 +359,12 @@ ol:focus li.selected .webkit-html-attribute-name {
     cursor: pointer;
 }
 
+@media (-webkit-min-device-pixel-ratio: 1.5) {
+.node-actions-toggle {
+    -webkit-mask-image: url(Images/toolbarButtonGlyphs_2x.png);
+}
+} /* media */
+
 .gutter-container.has-decorations .node-actions-toggle {
     opacity: 0;
 }