Use value, not pointer, comparison for generated StyleImages

Fixed crash caused by incorrect layout tree re-attachment of the box for
the anchored element during interleaved style recalc.

Bug: 419501749
Change-Id: I0336f8e040c40a3047541c5af7a17933ae71107f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6578277
Reviewed-by: Steinar H Gunderson 
Commit-Queue: Rune Lillesveen 
Cr-Commit-Position: refs/heads/main@{#1464649}

Author: lilles

css/css-anchor-position/chrome-419501749-crash.html

@@ -0,0 +1,12 @@
+>
+<link rel="help" href="https://crbug.com/419501749">
+<style>
+  #crash {
+    position: absolute;
+    left: anchor(right);
+    --grad: linear-gradient(black, blue);
+    content: var(--grad);
+  }
+style>
+<p>Pass if no crashp>
+<div id="crash">div>