Reland "[iOS] Start unified consent service at startup"

This is a reland of 3b42f46a

The original patch has been reverted with crrev.com/c/1162402.
The issue has been fixed with crrev.com/c/1163714. This patch
can be relanded.
The patch is to start the UnifiedConsentService at startup.

Original change's description:
> [iOS] Start unified consent service at startup
>
> Bug: 827072
> Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
> Change-Id: I2f2e14ceba73ebecbc9c0222ae87ea86e4f91a49
> Reviewed-on: https://chromium-review.googlesource.com/1159062
> Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
> Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
> Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#580485}

TBR: sdefresne
Bug: 827072
Change-Id: I3332f38fb24d4ba77e75a70d0201bae71b56cc67
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Reviewed-on: https://chromium-review.googlesource.com/1165062Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581220}

ios/chrome/browser/browser_state/BUILD.gn

@@ -116,6 +116,7 @@ source_set("browser_state_impl") {
     "//ios/chrome/browser/ui/overlays",
     "//ios/chrome/browser/ui/voice",
     "//ios/chrome/browser/undo",
+    "//ios/chrome/browser/unified_consent",
     "//ios/net",
     "//ios/public/provider/chrome/browser",
     "//ios/public/provider/chrome/browser/signin",

ios/chrome/browser/browser_state/chrome_browser_state_manager_impl.cc

@@ -41,6 +41,7 @@
 #include "ios/chrome/browser/signin/gaia_cookie_manager_service_factory.h"
 #include "ios/chrome/browser/signin/signin_manager_factory.h"
 #include "ios/chrome/browser/sync/profile_sync_service_factory.h"
+#include "ios/chrome/browser/unified_consent/unified_consent_service_factory.h"
 
 namespace {
 
@@ -223,6 +224,9 @@ void ChromeBrowserStateManagerImpl::DoFinalInitForServices(
   ios::AccountFetcherServiceFactory::GetForBrowserState(browser_state)
       ->SetupInvalidationsOnProfileLoad(invalidation_service);
   ios::AccountReconcilorFactory::GetForBrowserState(browser_state);
+  // Initialization needs to happen after the browser context is available
+  // because ProfileSyncService needs the URL context getter.
+  UnifiedConsentServiceFactory::GetForBrowserState(browser_state);
   DesktopPromotionSyncServiceFactory::GetForBrowserState(browser_state);
 }