Allow showing pending URL for new tab navigations, but only if safe.

Allow showing pending URL for new tab navigations, but only if safe.

We revert to showing the opener's URL if it modifies the content of the
initial blank page before the pending URL commits, to prevent URL spoofs.

Implications:
- All renderer-initiated navigations now have pending NavigationEntries.
- GetURL and GetTitle use the visible entry, not active entry.
- The renderer notifies the browser if DOM mutations occur before first load.

[Third attempt, after http://crrev.com/195553 was reverted.]

BUG=9682
TEST=Open a slow link in a new tab. Destination URL should be visible and reloadable.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=203732

[Charlie Reis, 2013-04-26 17:52:26]

Hi Darin and Tom--
  I'm preparing to land this fix for 9682 again.  It's the same as before, plus
the fix for http://crbug.com/234491 (which sky reviewed).  You can see that in
the diff between patch sets 1 and 2.

  I won't land this until the fix for http://crbug.com/234809 has landed, but I
think I'm close, and I want to have this part ready to go.

  Tom, can you approve view_messages.h again?  Darin, can you approve again?

Thanks!
Charlie

[Tom Sepez, 2013-04-26 18:48:07]

lgtm

[darin (slow to review), 2013-04-26 21:39:18]

LGTM

[commit-bot: I haz the power, 2013-06-03 16:53:41]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/creis@chromium.org/14283005/11001

[commit-bot: I haz the power, 2013-06-03 18:07:35]

Change committed as 203732