[NativeFS] Add some assertions for the locked state of streams.

https://github.com/WICG/native-file-system/issues/194 suggests there
might be something weird going here, so add assertions to the tests to
make sure locked has the value we expect it to have.

Change-Id: Ia13251aeeb16734047a2f5abe7e61d7fd4bbeb0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2285293
Auto-Submit: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786059}

third_party/blink/web_tests/external/wpt/native-file-system/script-tests/FileSystemWritableFileStream-write.js

@@ -295,7 +295,9 @@ directory_test(async (t, root) => {
 directory_test(async (t, root) => {
   const handle = await createEmptyFile(t, 'writer_written', root);
   const stream = await handle.createWritable();
+  assert_false(stream.locked);
   const writer = stream.getWriter();
+  assert_true(stream.locked);
 
   await writer.write('foo');
   await writer.write(new Blob(['bar']));

third_party/blink/web_tests/external/wpt/native-file-system/script-tests/FileSystemWritableFileStream.js

@@ -104,14 +104,20 @@ directory_test(async (t, root) => {
 directory_test(async (t, root) => {
   const handle = await createEmptyFile(t, 'contents', root);
   const stream = await handle.createWritable();
+  assert_false(stream.locked);
 
   stream.write('abc');
+  assert_false(stream.locked);
   stream.write('def');
+  assert_false(stream.locked);
   stream.truncate(9);
+  assert_false(stream.locked);
   stream.seek(0);
+  assert_false(stream.locked);
   stream.write('xyz');
+  assert_false(stream.locked);
   await stream.close();
 
   assert_equals(await getFileContents(handle), 'xyzdef\0\0\0');
   assert_equals(await getFileSize(handle), 9);
-}, 'commands are queued');
+}, 'commands are queued, stream is unlocked after each operation');