[OsSettingsLanguages] Update iron-icon color for disabled button

Use google-grey-600 which is the same color as the disabled text.

View: http://screen/QCzfch2vrvgJjuy

Bug: 1113439
Change-Id: I1cb748b864cc72e919a997b80a395dfeee670350
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2423813
Commit-Queue: My Nguyen <myy@chromium.org>
Reviewed-by: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810109}

chrome/browser/resources/settings/chromeos/os_languages_page/os_languages_page_v2.html

@@ -41,11 +41,15 @@
       }
 
       #addLanguages {
+        --iron-icon-fill-color: var(--cr-link-color);
         margin-top: 16px;
       }
 
+      #addLanguages[disabled] {
+        --iron-icon-fill-color: var(--google-grey-600);
+      }
+
       #addLanguagesIcon {
-        --iron-icon-fill-color: var(--cr-link-color);
         margin-inline-end: 8px;
       }
     </style>