core-suggest: inconsistent hidden attribute behaviour across browsers

[klizter]

Core-suggest hidden attribute behaves differently across browsers when not explicitly set.

Steps to reproduce:

1. Locate test does not open suggestions on clicking a disabled input
2. Remove the hidden attribute from <core-suggest> in page.setContent(...)
3. Run the test for all browsers

Expected result: All browsers should have the same assertion for core-suggest.hidden to be true

Actual result: Firefox asserts hidden to be false, while webkit and chromium asserts to be true