Linux: Update PRIMARY selection on omnibox copy-to-clipboard.

Linux: Update PRIMARY selection on omnibox copy-to-clipboard.

This brings us in line with Firefox's behavior.  Ctrl-L highlights
the URL but doesn't update the PRIMARY selection, so Ctrl-C is a
common* way to get the current location into PRIMARY so you can
middle-click somewhere else to paste it.

* I use it and a user mentioned that they do too. :-P

Tested as follows:

- highlight some text on the page to make it PRIMARY
- hit Ctrl-L and confirm that text on page is still PRIMARY
- hit Ctrl-C and confirm that location is now PRIMARY
- repeat, but this time hit the right arrow key between Ctrl-L
  and Ctrl-C to unhighlight the text first.  the text on the
  page should remain the PRIMARY selection

BUG=18403
TESTED=see above

[Evan Stade, 2009-08-14 00:33:37]

lgtm

[Daniel Erat, 2009-08-14 00:33:58]

Forgot to mention, but I intentionally didn't hook up the cut signal.  It seems
more arguable that we should update PRIMARY there, since the text is no longer
highlighted (or even present) after Ctrl-X.

[Dean McNamee, 2009-08-14 00:35:55]

Okay