Scale spinner with font size

With larger (or smaller font) the absolute pixel sizes for the spinner
change the relative center of the animation which leads to an uneven
rotation.

To prevent this, the spinner now has a size relative to the font size
which is identical for the current size on normal scaling but scales
with larger/smaller sizes to keep the animation intact.

This fix is inspired by crrev.com/c/2250203

Fixed: 1111300

Change-Id: I6626f107096cb0465efd109179c88cb88a54b7e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2337438
Commit-Queue: Varun Khaneja <vakh@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Auto-Submit: Varun Khaneja <vakh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794770}

chrome/browser/resources/settings/chrome_cleanup_page/chrome_cleanup_page.html

@@ -5,8 +5,8 @@
 
       #waiting-spinner {
         flex-shrink: 0;
-        height: 20px;
-        width: 20px;
+        height: 2.0em;
+        width: 2.0em;
       }
 
       /* Apply a fixed height to the <svg> tag inside #powered-by-logo.