[ios] Update Firebase library to 6.16

This brings in the new library files as a CIPD package and update
DEPS to refer to this updated library.

Fixed: 1049042
Change-Id: I874f1a148bd7c47a4756d25f5edb590a39709b0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042215Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739162}

DEPS

@@ -511,7 +511,7 @@ deps = {
       'packages': [
         {
           'package': 'chromium/third_party/firebase_ios',
-          'version': 'QmLncusdxHhHwXryoNEHHUGacx1Tg5ij449uwZL2dpAC',
+          'version': 'LF-Sg4wxHfilYCXiSZoj0gcLub1ueZX-q8H-xAVUrKQC',
         },
       ],
       'condition': 'checkout_ios',

ios/third_party/firebase/README.chromium

 Name: Firebase SDK
 Short Name: Firebase
 URL: https://firebase.google.com/terms/
-Version: 5.18.1
+Version: 6.16.0
 License: Custom License
 License File: LICENSE