[Merge-M110] [CrOS] MediaApp: Fix web share failures.

Starting on M110, third-party iframes need to use `allow: web-share` to
be able to use web share inside the iframe. Add that to the Gallery /
MediaApp iframe to fix being able to use web share in M110.

(cherry picked from commit 31a299f)

Bug: b/265748512
Change-Id: I7fd1c30e622dc7a96497d4ce27bba09fedead2bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4220773
Commit-Queue: Patti Lor <patricialor@chromium.org>
Reviewed-by: Callistus Tan <callistus@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#1100816}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4220919
Auto-Submit: Patti Lor <patricialor@chromium.org>
Commit-Queue: Callistus Tan <callistus@google.com>
Cr-Commit-Position: refs/branch-heads/5481@{#984}
Cr-Branched-From: 130f3e4-refs/heads/main@{#1084008}

ash/webui/media_app_ui/resources/index.html

@@ -31,6 +31,6 @@
 </style>
 <script src="chrome://resources/mojo/mojo/public/js/mojo_bindings_lite.js"></script>
 <script src="/first_message_received.js"></script>
-<iframe src="chrome-untrusted://media-app/app.html" allow="fullscreen; cross-origin-isolated;"></iframe>
+<iframe src="chrome-untrusted://media-app/app.html" allow="fullscreen; cross-origin-isolated; web-share;"></iframe>
 <script src="/launch.js" type="module"></script>
 </html>

ash/webui/media_app_ui/resources/index_dark_light.html

@@ -41,6 +41,6 @@
 </style>
 <script src="chrome://resources/mojo/mojo/public/js/mojo_bindings_lite.js"></script>
 <script src="/first_message_received.js"></script>
-<iframe src="chrome-untrusted://media-app/app.html" allow="fullscreen; cross-origin-isolated;"></iframe>
+<iframe src="chrome-untrusted://media-app/app.html" allow="fullscreen; cross-origin-isolated; web-share;"></iframe>
 <script src="/launch.js" type="module"></script>
 </html>