Make ::interest-button opt-in

For now, the debate continues:

w3c/csswg-drafts#13980

But in the meantime, there is quite strong developer frustration
with making `::interest-button` show up by default, even if it
is only on mobile screens. So for now, this CL makes the
`::interest-button` pseudo work like others: it only appears
if the developer gives it a `content: {something}` value.

Bug: 453705247
Change-Id: Ia4e3c05cc22c9f8454a0f807373ab3164d3898c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8021182
Auto-Submit: Mason Freed <masonf@chromium.org>
Reviewed-by: David Baron <dbaron@chromium.org>
Commit-Queue: David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1654751}

Author: mfreed7

html/semantics/interestfor/interestfor-pseudo-element-appearance-ref.html

@@ -30,12 +30,10 @@
 <div class="example defaultcontent">
   <button class=invoker>
     <span>Button</span>
-    <span class=fake-interest-button>&#x24D8;</span>
   </button>
 </div>
 <div class="example defaultcontent">
   <a href="#" class=invoker><span>Link</span>
-    <span class=fake-interest-button>&#x24D8;</span>
   </a>
 </div>