Make //content have no baseline permissions policy for isolated apps

Baseline permissions policy (aka high-watermark permissions policy) is
strictly an Isolated Web Apps concept for the //chrome layer -- it's not
exactly spec-ed and isn't a hard requirement for other embedders.

Allowing it to be optional outside of //chrome (and falling back to how
regular pages behave) eliminates the need to define approximate baseline
policies in WPTs and //content browser tests.

In addition this CL opts WPTs relying on isolated-context-origins into
proper process-level isolation instead of pretending that they're
running in embedder-defined IsolatedContext.

Bug: 340886976
Change-Id: Ia893e61efb372088228bd9bed3da2be4a7d2e17f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7594805
Reviewed-by: Zgroza (Luke) Klimek <zgroza@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Commit-Queue: Andrew Rayskiy <greengrape@google.com>
Auto-Submit: Andrew Rayskiy <greengrape@google.com>
Cr-Commit-Position: refs/heads/main@{#1594940}

Author: GrapeGreen

direct-sockets/tcp_socket.https.html.headers

@@ -1,4 +1,4 @@
 Cross-Origin-Opener-Policy: same-origin
 Cross-Origin-Embedder-Policy: require-corp
-Permissions-Policy: direct-sockets=(self)
+Permissions-Policy: cross-origin-isolated=(self), direct-sockets=(self)
 Origin-Agent-Cluster: ?0

direct-sockets/udp_socket.https.html.headers

@@ -1,4 +1,4 @@
 Cross-Origin-Opener-Policy: same-origin
 Cross-Origin-Embedder-Policy: require-corp
-Permissions-Policy: direct-sockets=(self)
+Permissions-Policy: cross-origin-isolated=(self), direct-sockets=(self)
 Origin-Agent-Cluster: ?0