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. 

[Second attempt, after http://crrev.com/191277 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=195553

[Charlie Reis, 2013-04-19 22:37:38]

Hi Darin and Tom--
  Can you approve this CL again?  Should be identical to
https://chromiumcodereview.appspot.com/12541018/, which was reverted.  I've
fixed the reason it was reverted in http://crbug.com/233053.

Thanks!
Charlie

[Tom Sepez, 2013-04-19 22:54:51]

Rubberstamp LGTM

[darin (slow to review), 2013-04-22 16:21:56]

LGTM

[commit-bot: I haz the power, 2013-04-22 16:32:12]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/creis@chromium.org/13846007/1

[commit-bot: I haz the power, 2013-04-22 18:04:06]

Change committed as 195553