Resolve inconsistency between browsers (#15800)

Resolve an inconsistency between Chrome and Firefox with how the
contentscript runs in an iframe.

This should have no user-facing impact, it's just meant as a safeguard
in case something unintentionally gets included in the contentscript.

app/manifest/v2/chrome.json

@@ -1,4 +1,5 @@
 {
+  "content_security_policy": "frame-ancestors 'none'",
   "externally_connectable": {
     "matches": ["https://metamask.io/*"],
     "ids": ["*"]

app/manifest/v3/chrome.json

@@ -1,4 +1,7 @@
 {
+  "content_security_policy": {
+    "extension_pages": "default-src 'self'; frame-ancestors 'none'"
+  },
   "externally_connectable": {
     "matches": ["https://metamask.io/*"],
     "ids": ["*"]