Remove the default backspace event handler.

We have UseCounters showing that 0.04% of page views navigate back
via the backspace button and 0.005% of page views are after a form
interaction. The latter are often cases where the user loses
data. Years of user complaints have been enough that we think
it's the right choice to change this given the degree of pain
users feel by losing their data and because every platform
has another keyboard combination that navigates back.

We're doing this via a flag so that we can control this behavior
should there be sufficient outcry.

Review URL:

Cr-Commit-Position: refs/heads/master@{#386433}
2 files changed