Remove MTP support on Linux.

components/storage_monitor/test_storage_monitor.h

Index: components/storage_monitor/test_storage_monitor.h
diff --git a/components/storage_monitor/test_storage_monitor.h b/components/storage_monitor/test_storage_monitor.h
index ec29b8c66c0d7f73fac35625cd39805dea4e4fe6..df3c57a4240cc3cd152407ac4847479040eb9838 100644
--- a/components/storage_monitor/test_storage_monitor.h
+++ b/components/storage_monitor/test_storage_monitor.h
@@ -46,7 +46,7 @@ class TestStorageMonitor : public StorageMonitor {
       base::string16* storage_object_id) const override;
 #endif
 
-#if defined(OS_LINUX)
+#if defined(OS_CHROMEOS)
   device::MediaTransferProtocolManager* media_transfer_protocol_manager()
       override;
 #endif
@@ -73,7 +73,7 @@ class TestStorageMonitor : public StorageMonitor {
   // Paths considered for testing purposes to be on removable storage.
   std::vector<base::FilePath> removable_paths_;
 
-#if defined(OS_LINUX)
+#if defined(OS_CHROMEOS)
   std::unique_ptr<device::MediaTransferProtocolManager>
       media_transfer_protocol_manager_;
 #endif