Convert NavigationEntry title to string16. TabContents::GetTitle no longer ne...

Convert NavigationEntry title to string16. TabContents::GetTitle no longer needs
to be virtual, either.

This also changes how the display URL is computed. Instead of doing it
preemptively, we now do so lazily. This allows us to do the URL formatting
correctly using the elider so that we can do IDN and unescaping.

I changed string_util's singleton functions. I was worried that other code
might make a singleton of string, which would give you this same value as a
non-const string. This would mean our empty strings might no longer be empty.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=10872