Remove the backspace/shift+backspace navigating the page.

This is part two of
https://crrev.com/a9fc5462ba084936191d51a9391ec5255e79a930.
Previously, EventHandler.cpp was doing the navigation
on Win/Mac and the code in chrome/browser was handling
Linux. So the previous UseCounter was correct for Win/Mac.

Review-Url: https://codereview.chromium.org/1922993002
Cr-Commit-Position: refs/heads/master@{#390283}
5 files changed