[web] Add tests for unified platform view embedding behavior (#172313)

Adds tests for expected behavior of platform view embedding for the
unified renderer frontend.

We currently skip the tests for the renderer which doesn't yet comport
to the expected unified behavior. See [this
doc](https://docs.google.com/document/d/1-BYZO_oAOJkS_spmELqCmnPQIEUibT3Sj2GhIL3luow/edit?tab=t.0)
for an overview of the different behaviors and the expected behavior of
the unified platform view embedder.

This adds tests for https://github.com/flutter/flutter/issues/172308, a
follow-up PR will unify the platform view embedder behavior and unskip
the tests for the misbehaving renderer.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md
https://dart.googlesource.com/external/github.com/flutter/flutter/+/b2d4210b3795413c2360968b685743a6df60ff50
diff --git a/DEPS b/DEPS
index a474968..453e0e2 100644
--- a/DEPS
+++ b/DEPS
@@ -231,7 +231,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '79cbfa032d220c5f7f9cb3753a69d6824bf1f78d',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'e2441683275879df1ac0311e02ff868410599ab1',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + 'b2d4210b3795413c2360968b685743a6df60ff50',
 
   'flutter/engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',
diff --git a/commits.json b/commits.json
index a5e9671..53af5cc 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"e2441683275879df1ac0311e02ff868410599ab1",
+  "flutter":"b2d4210b3795413c2360968b685743a6df60ff50",
   "flutter/engine/src/flutter/third_party/dart":"79cbfa032d220c5f7f9cb3753a69d6824bf1f78d"
 }
\ No newline at end of file