Only support seeking file streams from the beginning of the file.

net/base/file_stream_context.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/base/file_stream_context.h"
 
 #include "base/files/file_path.h"
 #include "base/location.h"
 #include "base/message_loop/message_loop_proxy.h"
 #include "base/profiler/scoped_tracker.h"
@@ skipping 98 matching lines @@
       FROM_HERE,
       base::Bind(&Context::CloseFileImpl, base::Unretained(this)),
       base::Bind(&Context::OnAsyncCompleted,
                  base::Unretained(this),
                  IntToInt64(callback)));
   DCHECK(posted);
 
   async_in_progress_ = true;
 }
 
-void FileStream::Context::Seek(base::File::Whence whence,
-                               int64 offset,
+void FileStream::Context::Seek(int64 offset,
                                const Int64CompletionCallback& callback) {
   DCHECK(!async_in_progress_);
 
   bool posted = base::PostTaskAndReplyWithResult(
-      task_runner_.get(),
-      FROM_HERE,
-      base::Bind(
-          &Context::SeekFileImpl, base::Unretained(this), whence, offset),
-      base::Bind(&Context::OnAsyncCompleted,
-                 base::Unretained(this),
-                 callback));
+      task_runner_.get(), FROM_HERE,
+      base::Bind(&Context::SeekFileImpl, base::Unretained(this), offset),
+      base::Bind(&Context::OnAsyncCompleted, base::Unretained(this), callback));
   DCHECK(posted);
 
   async_in_progress_ = true;
 }
 
 void FileStream::Context::Flush(const CompletionCallback& callback) {
   DCHECK(!async_in_progress_);
 
   bool posted = base::PostTaskAndReplyWithResult(
       task_runner_.get(),
       FROM_HERE,
       base::Bind(&Context::FlushFileImpl, base::Unretained(this)),
       base::Bind(&Context::OnAsyncCompleted,
                  base::Unretained(this),
                  IntToInt64(callback)));
   DCHECK(posted);
 
   async_in_progress_ = true;
 }
 
+bool FileStream::Context::IsOpen() const {
+  return file_.IsValid();
+}
+
 FileStream::Context::OpenResult FileStream::Context::OpenFileImpl(
     const base::FilePath& path, int open_flags) {
 #if defined(OS_POSIX)
   // Always use blocking IO.
   open_flags &= ~base::File::FLAG_ASYNC;
 #endif
   base::File file;
 #if defined(OS_ANDROID)
   if (path.IsContentUri()) {
     // Check that only Read flags are set.
@@ skipping 74 matching lines @@
   // should be reset before Close() because it shouldn't run if any async
   // operation is in progress.
   async_in_progress_ = false;
   if (orphaned_)
     CloseAndDelete();
   else
     callback.Run(result.result);
 }
 
 }  // namespace net