Make it clear that syncFS getrequestFileSystem can fail.

chrome/common/extensions/api/sync_file_system.idl

Index: chrome/common/extensions/api/sync_file_system.idl
diff --git a/chrome/common/extensions/api/sync_file_system.idl b/chrome/common/extensions/api/sync_file_system.idl
index c1e2eebef29be9c05d799324978e041a5b011617..4ff291209f8a096e412987643cf545b2deac37d8 100644
--- a/chrome/common/extensions/api/sync_file_system.idl
+++ b/chrome/common/extensions/api/sync_file_system.idl
@@ -146,6 +146,10 @@ namespace syncFileSystem {
     //
     // Calling this multiple times from
     // the same app will return the same handle to the same file system.
+    //
+    // Note this call can fail. For example, if the user is not signed in to
+    // Chrome or if there is no network operation. To handle these errors it is
+    // important chrome.runtime.lastError is checked in the callback.
     static void requestFileSystem(GetFileSystemCallback callback);
 
     // Sets the default conflict resolution policy