ui/base/models: Fix the API example code comment.

ui/base/models: Fix the API example code comment.

Allocating |root| on the stack and passing to |model|, also on the stack,
does _not_ work, because TreeNodeModel owns the root.
So when the dtor of TreeNodeModel is called, it attempts to free an invalid
pointer, which was already deleted.

BUG=None
TEST=None

R=sky@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=89355

[sky, 2011-06-16 16:06:45]

What's wrong with the example the way it was?

  -Scott

[tfarina, 2011-06-16 16:13:12]

On 2011/06/16 16:06:45, sky wrote:
> What's wrong with the example the way it was?
> 

If you try it as it's, as I tried in the unittests (compiles, but it won't
work), it tries to free |root|, but the memory was already freed. Also it's in
the commit message.

>   -Scott

[sky, 2011-06-16 16:26:39]

Good point. LGTM