[iOS] Enable support for finch experiment for Open from Clipboard.

BUG=474249

Review URL: https://codereview.chromium.org/1147843002

Cr-Commit-Position: refs/heads/master@{#330725}

ios/chrome/browser/experimental_flags.mm

@@ -31,7 +31,10 @@ bool IsAlertOnBackgroundUploadEnabled() {
 
 bool IsOpenFromClipboardEnabled() {
   base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
-  return command_line->HasSwitch(switches::kEnableIOSOpenFromClipboard);
+  if (command_line->HasSwitch(switches::kEnableIOSOpenFromClipboard))
+    return true;
+  return base::FieldTrialList::FindFullName("IOSOpenFromClipboard") ==
+         "Enabled";
 }
 
 bool IsWKWebViewEnabled() {