Add IDL and initial Blink API for Background Sync

Source/modules/background_sync/SyncManager.idl

Index: Source/modules/background_sync/SyncManager.idl
diff --git a/Source/bindings/tests/idls/core/TestPartialInterface4.idl b/Source/modules/background_sync/SyncManager.idl
similarity index 34%
copy from Source/bindings/tests/idls/core/TestPartialInterface4.idl
copy to Source/modules/background_sync/SyncManager.idl
index 524087cdbe82c7bb84404ffd80b87cabb483e262..7318d19d332f06a6d0b37ae1f33bd500f2a1628d 100644
--- a/Source/bindings/tests/idls/core/TestPartialInterface4.idl
+++ b/Source/modules/background_sync/SyncManager.idl
@@ -3,8 +3,12 @@
 // found in the LICENSE file.
 
 [
-    Conditional=FOO,
+    Exposed=(Window),
+    GarbageCollected,
+    RuntimeEnabled=BackgroundSync,
     TypeChecking=Interface,
-] partial interface TestInterface3 {
-    [Conditional=BAR, TypeChecking=Unrestricted] static void voidMethodDocument(Document document, double d);
+] interface SyncManager {
+    [CallWith=ScriptState,ImplementedAs=registerFunction] Promise<SyncRegistration> register(SyncRegistrationOptions options);
+    [CallWith=ScriptState] Promise<sequence<SyncRegistration>> getRegistrations();
+    readonly attribute unsigned long minAllowablePeriod;
 };