Use new size graphic for browser action icon graphic

Size is changed to 18px as per brave/brave-browser#2295

BUILD.gn

@@ -7,6 +7,12 @@ action("brave-extension") {
     "app/actions/tabActions.ts",
     "app/actions/webNavigationActions.ts",
     "app/actions/windowActions.ts",
+    "app/assets/img/shields-off.png",
+    "app/assets/img/shields-off@2x.png",
+    "app/assets/img/shields-off@3x.png",
+    "app/assets/img/shields-on.png",
+    "app/assets/img/shields-on@2x.png",
+    "app/assets/img/shields-on@3x.png",
     "app/background.ts",
     "app/background/actions/cosmeticFilterActions.ts",
     "app/background/actions/runtimeActions.ts",
@@ -103,8 +109,12 @@ action("brave-extension") {
     "$build_dir/img/icon-64.png",
     "$build_dir/img/icon-128.png",
     "$build_dir/img/icon-256.png",
-    "$build_dir/img/icon.svg",
-    "$build_dir/img/icon-off.svg",
+    "$build_dir/img/shields-off.png",
+    "$build_dir/img/shields-off@2x.png",
+    "$build_dir/img/shields-off@3x.png",
+    "$build_dir/img/shields-on.png",
+    "$build_dir/img/shields-on@2x.png",
+    "$build_dir/img/shields-on@3x.png",
     "$build_dir/_locales/en_US/messages.json",
   ]