Move content/child/background_sync to Blink.

third_party/WebKit/Source/modules/background_sync/SyncManager.h

Index: third_party/WebKit/Source/modules/background_sync/SyncManager.h
diff --git a/third_party/WebKit/Source/modules/background_sync/SyncManager.h b/third_party/WebKit/Source/modules/background_sync/SyncManager.h
index aba3118bc638329cbc1e426e72e756050810413a..0cf8708794361c19d3326b96ead5ca4c9305ea92 100644
--- a/third_party/WebKit/Source/modules/background_sync/SyncManager.h
+++ b/third_party/WebKit/Source/modules/background_sync/SyncManager.h
@@ -11,6 +11,7 @@
 
 namespace blink {
 
+class BackgroundSyncProvider;
 class ExecutionContext;
 class ScriptPromise;
 class ScriptState;
@@ -32,9 +33,13 @@ class SyncManager final : public GarbageCollected<SyncManager>,
 
   DECLARE_TRACE();
 
+  enum { kUnregisteredSyncID = -1 };
+
  private:
   explicit SyncManager(ServiceWorkerRegistration*);
 
+  static BackgroundSyncProvider* backgroundSyncProvider();
+
   Member<ServiceWorkerRegistration> m_registration;
 };