Remove modules/filesystem/DraggedIsolatedFileSystem.h dependency from core/DEPS.

Source/modules/filesystem/DataTransferItemFileSystem.cpp

Index: Source/modules/filesystem/DataTransferItemFileSystem.cpp
diff --git a/Source/modules/filesystem/DataTransferItemFileSystem.cpp b/Source/modules/filesystem/DataTransferItemFileSystem.cpp
index 50fc03306d6dbb9ab8330235a6010edf92395215..ab61d0365b3c8719c94252150d1cd01851f2ee26 100644
--- a/Source/modules/filesystem/DataTransferItemFileSystem.cpp
+++ b/Source/modules/filesystem/DataTransferItemFileSystem.cpp
@@ -60,15 +60,12 @@ PassRefPtrWillBeRawPtr<Entry> DataTransferItemFileSystem::webkitGetAsEntry(Execu
         return nullptr;
     ASSERT(file->isFile());
 
-    DraggedIsolatedFileSystem* filesystem = DraggedIsolatedFileSystem::from(item.clipboard()->dataObject().get());
-    DOMFileSystem* domFileSystem = filesystem ? filesystem->getDOMFileSystem(executionContext) : 0;
-    if (!filesystem) {
+    DOMFileSystem* domFileSystem = DraggedIsolatedFileSystem::getDOMFileSystem(item.clipboard()->dataObject().get(), executionContext);
+    if (!domFileSystem) {
         // IsolatedFileSystem may not be enabled.
         return nullptr;
     }
 
-    ASSERT(domFileSystem);
-
     // The dropped entries are mapped as top-level entries in the isolated filesystem.
     String virtualPath = DOMFilePath::append("/", toFile(file)->name());