Toolbar: Constrain the site icon's size.

The toolbar site icon added in 7.1 is sized only by `width` and `height`. A theme or plugin rule such as `img { width: auto !important; height: auto !important; }` overrides those, and the icon renders at the size of the image file. Add matching `max-width`, `max-height`, `min-width`, and `min-height`, and qualify the selector with `img` for added specificity, so the icon stays 20 pixels square, or 28 pixels on small screens.

Follow-up to r62625.

Props donniam, kgagne, khokansardar, jorbin, webverts, adrianduffell, iamchitti, joedolson, sabernhardt, adamsilverstein.
Fixes #65912.


git-svn-id: https://develop.svn.wordpress.org/trunk@64034 602fd350-edb4-49c9-b593-d223f7449a82

Author: whyisjake

src/wp-includes/css/admin-bar.css

@@ -590,9 +590,14 @@ html:lang(he-il) .rtl #wpadminbar * {
 	content: none;
 }
 
-#wpadminbar #wp-admin-bar-site-name > .ab-item .site-icon {
+/* The adminbar icon needs additional styling to prevent 3rd party styles from overriding the size. */
+#wpadminbar #wp-admin-bar-site-name > .ab-item img.site-icon {
 	width: 20px;
 	height: 20px;
+	max-width: 20px;
+	max-height: 20px;
+	min-width: 20px;
+	min-height: 20px;
 	border-radius: 2px;
 }
 
@@ -942,12 +947,16 @@ html:lang(he-il) .rtl #wpadminbar * {
 		display: block;
 	}
 
-	#wpadminbar #wp-admin-bar-site-name > .ab-item .site-icon {
+	#wpadminbar #wp-admin-bar-site-name > .ab-item img.site-icon {
 		position: absolute;
 		top: 9px;
 		left: 12px;
 		width: 28px;
 		height: 28px;
+		max-width: 28px;
+		max-height: 28px;
+		min-width: 28px;
+		min-height: 28px;
 		margin: 0;
 		border-radius: 4px;
 	}