Avoid suggesting the empty string for navigator.platform

navigator.platform was permitted to return the empty string due to a privacy-motivated spec change in ecefa6e.

Since 2008, the understanding of changing things like this for privacy has evolved from returning special privacy values to returning a value that is already commonly returned for non-privacy reasons. Therefore, change the spec not to say "the empty string" but to permit returning a string commonly returned on another platform.

Additionally, change the examples to be from the set of values given at https://www.chromium.org/updates/ua-reduction/ to avoid suggesting more-identifying and potentially-less-compatible values. But, for now, avoid actually constraining the permitted values to a reduced set.

Further work on constraining these values is tracked mostly in https://github.com/whatwg/compat; see especially whatwg/compat#174.

source

@@ -98592,9 +98592,10 @@ interface <dfn interface>Navigator</dfn> {
    <!-- oscpu: Mozilla only -->
 
    <dt><dfn attribute for="NavigatorID"><code data-x="dom-navigator-platform">platform</code></dfn></dt>
-   <dd><p>Must return either the empty string or a string representing the platform on which the
-   browser is executing, e.g. "<code data-x="">MacIntel</code>", "<code data-x="">Win32</code>",
-   "<code data-x="">FreeBSD i386</code>", "<code data-x="">WebTV OS</code>".</p></dd>
+   <dd><p>Must return a string representing the platform on which the
+   browser is executing (e.g. "<code data-x="">MacIntel</code>", "<code data-x="">Win32</code>",
+   "<code data-x="">Linux x86_64</code>", "<code data-x="">Linux armv81</code>") or, for privacy and
+   compatibility, a string that is commonly returned on another platform.</p></dd>
 
    <dt><dfn attribute for="NavigatorID"><code data-x="dom-navigator-product">product</code></dfn></dt>
    <dd><p>Must return the string "<code data-x="">Gecko</code>".</p></dd>