Linux: terminate clipboard handling on the UI thread.

Linux: terminate clipboard handling on the UI thread.

After discussing with various people, I think we're going to do this
in the short term at least. Currently we're calling into GTK from the
IO thread and Bad Things Happen when one does that.

We would like to write an Xlib, fully asynchronous system for dealing
with the clipboard (and to get the clipboard stuff out of base!). That
would let us avoid sending the selection over the IPC channel each
time it updates too.

However, that's going to be a lot of work and we have crashing
browsers happening /right now/. Also, Evan thinks that maybe we don't
have the deadlock situation on Linux that we do on Windows with
terminating sync requests from the renderer on the UI thread.

[Evan Martin, 2009-05-01 01:10:55]

typo in review desc: discuessing

I kinda don't understand the relative lifetimes and locks of the threads
involved here.  E.g. we have code on the IO thread that calls ui_loop().

However, if you're confident this works (even in the case of shutdown, where
some threads might be available), then it LGTM.