Auto merge of #12406 - izgzhen:refactor-file, r=Manishearth

Refactor FileAPI implementation

Most are simple refactoring, cleanups and improvements, but still involving two slightly notable changes:

+ In `filemanager`, now we read the file content based on requested `RelativePos` by `seek` and `read_exact` (rather than `read_to_end` then do slicing). This strategy might be again adjusted in future performance tuning but certainly better than nothing.
+ Also, I cached more file meta-info in both sides and left a block of comment on `filemanager`'s file reading mentioning the snapshot-state problem (not solved now though).

r? @Manishearth

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components/script/dom/bindings/trace.rs

@@ -79,6 +79,7 @@ use std::collections::{BTreeMap, HashMap, HashSet};
 use std::hash::{BuildHasher, Hash};
 use std::mem;
 use std::ops::{Deref, DerefMut};
+use std::path::PathBuf;
 use std::rc::Rc;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, AtomicUsize};
@@ -331,6 +332,7 @@ no_jsmanaged_fields!(SystemTime);
 no_jsmanaged_fields!(SelectedFileId);
 no_jsmanaged_fields!(RelativePos);
 no_jsmanaged_fields!(OpaqueStyleAndLayoutData);
+no_jsmanaged_fields!(PathBuf);
 no_jsmanaged_fields!(CSSErrorReporter);
 no_jsmanaged_fields!(WebGLBufferId);
 no_jsmanaged_fields!(WebGLFramebufferId);