OS Settings WebUI: fix printer add icon color

--iron-icon-fill-color was being set directly for a <cr-icon-button>.
Even though the <cr-icon-button> currently uses a <iron-icon> which
defines and uses --iron-icon-fill-color, variables of components
contained within a component should not be used directly.

If there is <cr-icon-button> CSS variable what was is needed, that
should be used (which in this case there is one
--cr-icon-button-fill-color). If one does not exist, I think it makes
sense to create one and use that internally to set the CSS variable for
the contained element.

Bug: 1027466
Change-Id: I317a0dff63af9c5000596a16c6bc3038492d6c3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929105Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Esmael Elmoslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718321}

chrome/browser/resources/settings/printing_page/cups_printers.html

@@ -47,7 +47,7 @@
       }
 
       #addManualPrinterIcon {
-        --iron-icon-fill-color: rgb(138, 180, 248);
+        --cr-icon-button-fill-color: rgb(138, 180, 248);
         --cr-icon-button-margin-end: 0;
       }