Treat SVG documents in <object>/<embed> as replaced elements

SVG documents loaded inside <object> or <embed> elements were being
treated as regular iframe content rather than as replaced elements. As a
result, CSS properties like object-fit and object-position were being
ignored.

This CL modifies LayoutEmbeddedContent::ReplacedContentRectFrom() to
detect SVG documents and apply the replaced element logic of their base
class, making SVG content behave like images.

The test external/wpt/fullscreen/rendering/backdrop-object.html started
failing as a result of this CL, because the default value of
object-position is not what the reference assumed. Since this
functionality is outside the focus of that test, the solution was to
explicitly align object-position with the expectations of the reference.

Bug: 40747033, 41302806
Change-Id: I8681636a18232127df67e899e6fc1070864f68cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6821135
Reviewed-by: Koji Ishii <[email protected]>
Reviewed-by: Kent Tamura <[email protected]>
Commit-Queue: Felipe Erias <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1498641}

Author: felipeerias

fullscreen/rendering/backdrop-object.html

@@ -10,6 +10,9 @@
 object::backdrop {
   background: blue;
 }
+object {
+  object-position: 0 0;
+}
 </style>
 <object width="200" type="image/svg+xml"></object>