Remove the ability of webpages to specify strings for the onbeforeunload dialog.

BUG=587940
TEST=as in bug
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/1714573002

Cr-Commit-Position: refs/heads/master@{#380755}
44 files changed