Avoid using UserData in //content/browser/browser_context.cc

Before this CL, //content/browser/browser_context.cc would rely on
base::SupportsUserData to simulate 10+ fields of BrowserContext.  The
motivation for this was to hide //content-internal details (e.g. the
fields and their types) from //content/public API (e.g. from the
declaration of the BrowserContext class).

After this CL, explicit fields are used while still being hidden from
//content/public API behind a private, fwd-declared Impl class.

Motivation:

- Removing the SupportsUserData dependency is a necessary step toward
  migration to separate BrowserContext, BrowserContextImpl and
  BrowserContextDelegate classes.

- Explicit fields are easier to read and use (when coding or when
  using a debugger).

Bug: 1179776
Change-Id: Ifdc3ff1d7b945678fa0a915af780c9631c998058
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2706279
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#868166}

content/browser/BUILD.gn

@@ -519,6 +519,8 @@ source_set("browser") {
     "browser_child_process_host_impl.h",
     "browser_child_process_host_impl_receiver_bindings.cc",
     "browser_context.cc",
+    "browser_context_impl.cc",
+    "browser_context_impl.h",
     "browser_interface_binders.cc",
     "browser_interface_binders.h",
     "browser_interface_broker_impl.h",

content/browser/background_sync/background_sync_scheduler.cc

@@ -7,16 +7,14 @@
 #include <algorithm>
 
 #include "base/memory/scoped_refptr.h"
-#include "base/supports_user_data.h"
+#include "content/browser/browser_context_impl.h"
 #include "content/browser/storage_partition_impl.h"
 #include "content/public/browser/background_sync_controller.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/browser_thread.h"
 
 namespace content {
 
-const char kBackgroundSyncSchedulerKey[] = "background-sync-scheduler";
-
 using DelayedProcessingInfoMap =
     std::map<StoragePartitionImpl*, std::unique_ptr<base::OneShotTimer>>;
 
@@ -26,17 +24,7 @@ BackgroundSyncScheduler* BackgroundSyncScheduler::GetFor(
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   DCHECK(browser_context);
 
-  if (!browser_context->GetUserData(kBackgroundSyncSchedulerKey)) {
-    scoped_refptr<BackgroundSyncScheduler> scheduler =
-        base::MakeRefCounted<BackgroundSyncScheduler>();
-    browser_context->SetUserData(
-        kBackgroundSyncSchedulerKey,
-        std::make_unique<base::UserDataAdapter<BackgroundSyncScheduler>>(
-            scheduler.get()));
-  }
-
-  return base::UserDataAdapter<BackgroundSyncScheduler>::Get(
-      browser_context, kBackgroundSyncSchedulerKey);
+  return browser_context->impl()->background_sync_scheduler();
 }
 
 BackgroundSyncScheduler::BackgroundSyncScheduler() = default;

content/browser/background_sync/background_sync_scheduler.h

@@ -21,9 +21,6 @@ namespace content {
 
 class StoragePartitionImpl;
 
-// Key name on BrowserContext.
-extern const char kBackgroundSyncSchedulerKey[];
-
 // This contains the logic to schedule delayed processing of (periodic)
 // Background Sync registrations.
 // It keeps track of all storage partitions, and the soonest time we should