Bug 1385005 - Put toolbarbutton-dropdown-arrow.png on the non-Photon browser_startup_images.js whitelist. r=johannh

This was originally removed from the whitelist when bug 1372689 landed. With
Photon enabled, this image doesn't load at all. Instead of trying to debug
the non-Photon case, we're whitelisting this again because Photon is going
out the door in the next release anyways.

MozReview-Commit-ID: DdN38s3oqST

--HG--
extra : rebase_source : 986c338fb8448f26719ecefd7c4a72db19614622

browser/base/content/test/performance/browser_startup_images.js

@@ -31,6 +31,13 @@ const whitelist = [
     photon: true,
   },
 
+  // Non-Photon-only entries
+  {
+    file: "chrome://browser/skin/toolbarbutton-dropdown-arrow.png",
+    platforms: ["linux", "win", "macosx"],
+    photon: false,
+  },
+
   // Shared entries
   {
     file: "chrome://browser/skin/arrow-left.svg",