Fix InvalidateFullScreen() for the full-screen-polling case.

BUG=83640
TEST=Manual

Review URL: http://codereview.chromium.org/6992007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@86354 0039d316-1c4b-4281-b951-d872f2087c98

remoting/host/capturer_linux.cc

@@ -263,6 +263,7 @@ void CapturerLinux::InvalidateScreen(const gfx::Size& size) {
 
 void CapturerLinux::InvalidateFullScreen() {
   helper_.InvalidateFullScreen();
+  last_buffer_ = NULL;
 }
 
 void CapturerLinux::CaptureInvalidRects(