Devtools [CustomFormatters]: Fix appearance of an arrow on the mac

BUG=

Review URL: https://codereview.chromium.org/1326953002

git-svn-id: svn://svn.chromium.org/blink/trunk@201737 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/devtools/front_end/components/customPreviewSection.css

@@ -22,6 +22,13 @@
     text-shadow: none;
 }
 
+@media (-webkit-min-device-pixel-ratio: 1.5) {
+    .custom-expandable-section-header::before {
+        background-image: url(Images/toolbarButtonGlyphs_2x.png);
+    }
+} /* media */
+
+
 .custom-expandable-section-header::before {
     background-position: -4px -96px;
 }