Fix broken iOS dark theme transition

will-change property causes the page background not to respond to the
background colour change. The CL scopes that property away from iOS
devices.

Bug: 1008776
Change-Id: I4bc2bd8fde0e9f60389be8be55108ad77b8bf8b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834228Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Edward Jung (EMEA) <edwardjung@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703253}

components/neterror/resources/neterror.css

@@ -791,7 +791,9 @@ TODO(https://crbug.com/852872): UI for offline suggested content is incomplete.
 .offline {
   transition: filter 1.5s cubic-bezier(0.65, 0.05, 0.36, 1),
               background-color 1.5s cubic-bezier(0.65, 0.05, 0.36, 1);
+<if expr="not is_ios">
   will-change: filter, background-color;
+</if>
 }
 
 .offline body {