Ignore Javascript urls dropped on tabs

When a Javascript: url is dropped on a tab, it executes in the
security context of the selected tab, representing a script
injection attack ("Dropjacking"). We will match other browsers
and disallow such drops.

BUG=639750

Review-Url: https://codereview.chromium.org/2323273003
Cr-Commit-Position: refs/heads/master@{#418620}
1 file changed