[FSA] Allow seeking past the end of a file

Currently, we reject all attempts to write past the end of a file. This
change makes the API more consistent with the behavior of POSIX.

If writing at an offset past the end of a file, the null bytes between
the old end of the file and the offset will count towards the file's
quota.

Adds Quota tests to the SandboxFSW.

Bug: 1153385
Change-Id: Iec54acbccecb728f9375931825525ba2fbafd1ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593810
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/master@{#850838}

file-system-access/script-tests/FileSystemWritableFileStream-write.js

@@ -117,14 +117,12 @@ directory_test(async (t, root) => {
   const handle = await createEmptyFile(t, 'bad_offset', root);
   const stream = await handle.createWritable();
 
-  await promise_rejects_dom(
-      t, 'InvalidStateError', stream.write({type: 'write', position: 4, data: new Blob(['abc'])}));
-  await promise_rejects_js(
-      t, TypeError, stream.close(), 'stream is already closed');
+  await stream.write({type: 'write', position: 4, data: new Blob(['abc'])});
+  await stream.close();
 
-  assert_equals(await getFileContents(handle), '');
-  assert_equals(await getFileSize(handle), 0);
-}, 'write() called with an invalid offset');
+  assert_equals(await getFileContents(handle), '\0\0\0\0abc');
+  assert_equals(await getFileSize(handle), 7);
+}, 'write() called with an offset beyond the end of the file');
 
 directory_test(async (t, root) => {
   const handle = await createEmptyFile(t, 'empty_string', root);