Don't allow drops of invalid URLs.

Don't allow drops of invalid URLs.

BUG=528228
TEST=none

Committed: https://crrev.com/2edf360e776b15161e7f11c07b28f64d2a99d7c7
Cr-Commit-Position: refs/heads/master@{#351115}

[Avi (use Gerrit), 2015-09-28 18:52:20]

The CQ bit was checked by avi@chromium.org to run a CQ dry run

[Avi (use Gerrit), 2015-09-28 18:53:03]

avi@chromium.org changed reviewers:
+ creis@chromium.org

[commit-bot: I haz the power, 2015-09-28 18:57:50]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1374793002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1374793002/1

[Charlie Reis, 2015-09-28 19:16:34]

LGTM.  Better than navigating to about:blank, which is what we do in 47.0.2521.0
now that the crash is gone.

[Avi (use Gerrit), 2015-09-28 19:18:57]

The CQ bit was unchecked by avi@chromium.org

[Avi (use Gerrit), 2015-09-28 19:19:00]

The CQ bit was checked by avi@chromium.org

[commit-bot: I haz the power, 2015-09-28 19:19:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1374793002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1374793002/1

[commit-bot: I haz the power, 2015-09-28 19:32:25]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-28 19:33:46]

Patchset 1 (id:??) landed as
https://crrev.com/2edf360e776b15161e7f11c07b28f64d2a99d7c7
Cr-Commit-Position: refs/heads/master@{#351115}