This makes an upstream DCHECK happy in AppIndicatorIcon::SetToolTip.
Empty tooltip strings are discouraged, as discussed in commit log
b6c510aa543193337041517c2d70113840189b06.
Fixes#12386.
The gfx::Image can use NSImage directly as underlying format, so we
don't have to decode images ourselves on Mac, and we will also be able
to make use of template images.
Now we only uses GTK+ for app indicator and status icon, also notice the
in Chrome35 the Gtk2StatusIcon class doesn't exist yet so we shipp
related files under chromium_src, remember to delete them after upgraded
to Chrome36.
