Bug 1853652 - Use image-set more in toolkit. r=desktop-theme-reviewers,dao

Depends on D188450

Differential Revision: https://phabricator.services.mozilla.com/D188451

toolkit/components/printing/content/print.css

@@ -179,19 +179,16 @@ input[type="text"] {
 }
 
 #print-progress {
-  background-image: url("chrome://global/skin/icons/loading.png");
+  background-image: image-set(
+    url("chrome://global/skin/icons/loading.png"),
+    url("chrome://global/skin/icons/loading@2x.png") 2x
+  );
   background-repeat: no-repeat;
   background-size: 16px;
   background-position: left center;
   padding-inline-start: 20px;
 }
 
-@media (min-resolution: 1.1dppx) {
-  #print-progress {
-    background-image: url("chrome://global/skin/icons/loading@2x.png");
-  }
-}
-
 #print-progress:dir(rtl) {
   background-position-x: right;
 }

toolkit/crashreporter/content/crashes.css

@@ -49,18 +49,15 @@ th,
 }
 
 .submitting {
-  background-image: url(chrome://global/skin/icons/loading.png);
+  background-image: image-set(
+    url(chrome://global/skin/icons/loading.png),
+    url(chrome://global/skin/icons/loading@2x.png) 2x
+  );
   background-position: center;
   background-repeat: no-repeat;
   background-size: 16px;
 }
 
-@media (min-resolution: 1.1dppx) {
-  .submitting {
-    background-image: url(chrome://global/skin/icons/loading@2x.png);
-  }
-}
-
 .submitting .submit-crash-button-label {
   display: none;
 }