Fix Chromium support for content property alt text (#6001)

It looks like the values for this feature were copied from the parent
feature. Upon a little digging, this feature appears to have landed in
Chromium 77.

https://www.chromestatus.com/features/4550056227110912

css/properties/content.json

@@ -53,10 +53,10 @@
             "description": "Alternative text after <code>/</code>",
             "support": {
               "chrome": {
-                "version_added": "1"
+                "version_added": "77"
               },
               "chrome_android": {
-                "version_added": "18"
+                "version_added": "77"
               },
               "edge": {
                 "version_added": "79"
@@ -71,10 +71,10 @@
                 "version_added": false
               },
               "opera": {
-                "version_added": "15"
+                "version_added": "64"
               },
               "opera_android": {
-                "version_added": "14"
+                "version_added": "55"
               },
               "safari": {
                 "version_added": false
@@ -83,10 +83,10 @@
                 "version_added": false
               },
               "samsunginternet_android": {
-                "version_added": "1.0"
+                "version_added": false
               },
               "webview_android": {
-                "version_added": "1"
+                "version_added": "77"
               }
             },
             "status": {