Fix clearing the findbar

[cndouglas]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

1. Navigate to any page and open the findbar.
2. Type in any word on the current page.
3. Click the clear button.
4. Make sure 1) the text field is empty, 2) the number of matches is gone, and 3) the highlighted terms found on the page are gone.

Fixes #5247

[bbondy]

++