Auto merge of #12579 - izgzhen:fix-fileapi-ref, r=Manishearth

Fix FileAPI's refcount implementation

Revise several intricate parts of FileAPI's internal refcounting-related implementation.

Goal: Get it done right once and for all.

r? @Manishearth

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because it is internal logic change

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12579)
<!-- Reviewable:end -->

components/net_traits/filemanager_thread.rs

@@ -138,18 +138,15 @@ pub enum FileManagerThreadMsg {
     /// as part of a valid Blob URL
     AddSlicedURLEntry(SelectedFileId, RelativePos, IpcSender<Result<SelectedFileId, BlobURLStoreError>>, FileOrigin),
 
-    /// Revoke Blob URL and send back the acknowledgement
-    RevokeBlobURL(SelectedFileId, FileOrigin, IpcSender<Result<(), BlobURLStoreError>>),
-
     /// Decrease reference count and send back the acknowledgement
     DecRef(SelectedFileId, FileOrigin, IpcSender<Result<(), BlobURLStoreError>>),
 
-    /// Increase reference count
-    IncRef(SelectedFileId, FileOrigin),
-
     /// Activate an internal FileID so it becomes valid as part of a Blob URL
     ActivateBlobURL(SelectedFileId, IpcSender<Result<(), BlobURLStoreError>>, FileOrigin),
 
+    /// Revoke Blob URL and send back the acknowledgement
+    RevokeBlobURL(SelectedFileId, FileOrigin, IpcSender<Result<(), BlobURLStoreError>>),
+
     /// Shut down this thread
     Exit,
 }