Fix compilation of official build with Firebase enabled

Official build with Firebase enabled uses variants to specify
which Info.plist to use as resource for Firebase. This causes
a conflict as two bundle_data have the same output path.

Move the dependency on ios_firebase_resources_target to the
definition of the "chrome" target, behind the check for use
of the variants to prevent this conflict.

Patchset 1 is the original CL http://crrev/c/1250924 by sdefresne
from which this CL is cloned from.

TBR=sdefresne@chromium.org, justincohen@chromium.org

Bug: 845383
Change-Id: I0e74f04f27708eb970c61b1048cb2492c45cbba4
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Reviewed-on: https://chromium-review.googlesource.com/1252321
Commit-Queue: Peter Lee <pkl@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595187}

ios/chrome/app/BUILD.gn

@@ -286,7 +286,20 @@ ios_app_bundle("chrome") {
 
   bundle_deps = [ "//ios/chrome/app/resources" ]
   if (!is_chrome_branded || ios_chrome_app_variants == []) {
+    assert(ios_application_icons_target != "",
+           "ios_application_icons_target must be defined.")
     bundle_deps += [ ios_application_icons_target ]
+
+    if (ios_enable_firebase_sdk) {
+      assert(ios_firebase_resources_target != "",
+             "ios_firebase_resources_target must be defined if Firebase SDK " +
+                 "is enabled.")
+
+      # Empty list assigned to bundle_deps to indicate that this is an
+      # intentional override of bundle_deps.
+      bundle_deps = []
+      bundle_deps = [ ios_firebase_resources_target ]
+    }
   } else {
     variants = ios_chrome_app_variants
   }

ios/third_party/firebase/BUILD.gn

@@ -2,13 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//ios/third_party/firebase/firebase.gni")
-
 source_set("firebase") {
-  assert(
-      ios_firebase_resources_target != "",
-      "ios_firebase_resources_target must be defined if Firebase SDK is enabled.")
-
   # From gn documentation:
   #   https://chromium.googlesource.com/chromium/src/+/master/tools/gn/docs/reference.md#ldflags
   # "ldflags are NOT pushed to dependents, so applying ldflags to source sets
@@ -19,9 +13,6 @@ source_set("firebase") {
   # This source_set must be specified as a direct deps of an ios_app_bundle
   # target for ldflags specified in :firebase_config to be applied.
   public_configs = [ ":firebase_config" ]
-  deps = [
-    ios_firebase_resources_target,
-  ]
 }
 
 config("firebase_config") {