[FileSearch] Can't iterate through search results after switching to a different file

[jasonLaster]

From: https://bugzilla.mozilla.org/show_bug.cgi?id=1384933

[Steps to reproduce]:

1. Go to https://devtools-html.github.io/debugger-examples/examples/todomvc/
2. Open the Debugger.
3. From "Source list pane" section, select a file that has the ".js" termination.
4. On Windows - Press CTRL + F
   On Mac - Press CMD + F
   Search for a term that can be found (e.g listenTo).
5. Navigate to another file (e.g app-view.js).
6. Press Enter to iterate forward or press Shift+Enter to iterate back.

[Expected result]:
Can iterate through search results after switching to a different file.

[Actual result]:
Can't iterate through search results for a different file.