Remove command line/field trial support and configs for Isolate Extensions.

chrome/browser/browser_process_impl.cc

Index: chrome/browser/browser_process_impl.cc
diff --git a/chrome/browser/browser_process_impl.cc b/chrome/browser/browser_process_impl.cc
index a0d1638fd5f2b85bb1981b1b0be0f4c7faf05893..850f3c6763479fc01eb06edbaa9c58546ad498e4 100644
--- a/chrome/browser/browser_process_impl.cc
+++ b/chrome/browser/browser_process_impl.cc
@@ -1078,18 +1078,10 @@ void BrowserProcessImpl::CreateLocalState() {
 
 void BrowserProcessImpl::PreCreateThreads() {
 #if BUILDFLAG(ENABLE_EXTENSIONS)
-  // Register the chrome-extension scheme to reflect the extension process
-  // model. Controlled by a field trial, so we can't do this earlier.
-  base::FieldTrialList::FindFullName("SiteIsolationExtensions");
-  if (extensions::IsIsolateExtensionsEnabled()) {
-    // chrome-extension:// URLs are safe to request anywhere, but may only
-    // commit (including in iframes) in extension processes.
-    ChildProcessSecurityPolicy::GetInstance()->RegisterWebSafeIsolatedScheme(
-        extensions::kExtensionScheme, true);
-  } else {
-    ChildProcessSecurityPolicy::GetInstance()->RegisterWebSafeScheme(
-        extensions::kExtensionScheme);
-  }
+  // chrome-extension:// URLs are safe to request anywhere, but may only
+  // commit (including in iframes) in extension processes.
+  ChildProcessSecurityPolicy::GetInstance()->RegisterWebSafeIsolatedScheme(
+      extensions::kExtensionScheme, true);
 #endif
 
   io_thread_.reset(