Move `externally_connectable` from base to Chrome manifest (#9824)

The `externally_connectable` property of the extension manifest is not
recognized by Firefox. It has been moved from the base manifest to the
Chrome manifest, so that we no longer get a warning about this property
on Firefox.

We would like to eventually remove it from the Chrome manifest as well,
but we'll wait until we can batch it with other permission changes so
that it doesn't unnecessarily re-prompt the user (see #9804)

app/manifest/_base.json

@@ -41,10 +41,6 @@
   ],
   "default_locale": "en",
   "description": "__MSG_appDescription__",
-  "externally_connectable": {
-    "matches": ["https://metamask.io/*"],
-    "ids": ["*"]
-  },
   "icons": {
     "16": "images/icon-16.png",
     "19": "images/icon-19.png",

app/manifest/chrome.json

@@ -1,3 +1,7 @@
 {
+  "externally_connectable": {
+    "matches": ["https://metamask.io/*"],
+    "ids": ["*"]
+  },
   "minimum_chrome_version": "58"
 }