ToggleBold does not work in specific editable contents

ToggleBold does not work in specific editable contents

[Title] ToggleBold does not work in specific editable contents
[Issue] P130610-4137
[Problem] In Email, ToggleBold does not work after 'select all' for some mails.
[Cause] The invisible text elements was included when checking triStateOfStyle.
[Solution] Add a checking routine for 'node->renderer' so that the invisible elements can't make the state as 'mixed'.
Added on opensource: http://webkit.org/b/118285

Change-Id: I721599d62b87a07bd39ca4a124c5d3e6f4059575