Fix the incorrect font used by buttons in ChromeOS.

WebUI should override the default Blink style for buttons so that buttons can be rendered with the correct font.

BUG=448624
TEST=manually checked on devices (Pixel)

Review URL: https://codereview.chromium.org/896673002

Cr-Commit-Position: refs/heads/master@{#314359}

ui/webui/resources/css/text_defaults.css

@@ -23,3 +23,7 @@ body {
   font-family: $2;
   font-size: $3;
 }
+
+button {
+  font-family: $2;
+}