Add block-start/end and overflow properties to popup

These were missing - see [1]. Without at least the block start
and end positions, pop-ups could easily be shown off screen below
the viewport.

[1] openui/open-ui#561 (comment)

Bug: 1307772
Change-Id: I006c664f89e8996d75c74130e4510967acfd486b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3803128
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Joey Arhar <jarhar@chromium.org>
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1030616}

html/semantics/popups/resources/popup-styles.css

@@ -2,20 +2,23 @@
   position: fixed;
   inset-inline-start: 0;
   inset-inline-end: 0;
+  inset-block-start: 0;
+  inset-block-end: 0;
   width: fit-content;
   height: fit-content;
   margin: auto;
   border: solid;
+  overflow: auto;
   padding: 1em;
-  background-color: Canvas;
   color: CanvasText;
+  background-color: Canvas;
 }
 .fake-pop-up-backdrop {
   position: fixed;
-  top: 0;
-  right: 0;
-  bottom: 0;
-  left: 0;
+  inset-inline-start: 0;
+  inset-inline-end: 0;
+  inset-block-start: 0;
+  inset-block-end: 0;
   background: transparent;
   pointer-events: none !important;
 }