[GTK] Fix for muted tooltips while sound is playing.

Tab media indicator data model updates indirectly cause TabGtk::UpdateTooltipState() to be invoked several times per second.  This in turned resulted in calling gtk_widget_set_tooltip_text() redundantly.  GTK responds to any tooltip text setter calls by hiding *all* tooltips throughout the application.

The fix is to check whether the tooltip text actually is different, and not call the setter when the text is unchanged.

In the bigger picture, there are other times when normal browser functionality would cause UpdateTooltipState() to be called several times per second.  In general, the current implementation for handling tab data model updates is expensive and in bad need of rewriting.  However, efforts would be better spent on moving Linux Chrome over to Aura.

BUG=333002
TEST=Confirmed fixed using bug repro steps.
R=erg@chromium.org

Review URL: https://codereview.chromium.org/138413006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245103 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245103