Clear root window when the aura root window is moved/resized

ui/aura/root_window_host_linux.cc

Index: ui/aura/root_window_host_linux.cc
diff --git a/ui/aura/root_window_host_linux.cc b/ui/aura/root_window_host_linux.cc
index 1f8cc0d1431cf0e3bcd670b3ca9694ebfa67b05a..88a8d0d24c5afe5758c96c16f5c491d40021eec1 100644
--- a/ui/aura/root_window_host_linux.cc
+++ b/ui/aura/root_window_host_linux.cc
@@ -511,6 +511,17 @@ bool RootWindowHostLinux::Dispatch(const base::NativeEvent& event) {
         delegate_->OnHostResized(bounds.size());
       if (origin_changed)
         delegate_->OnHostMoved(bounds_.origin());
+      // TODO(oshima): Clear the root when the window is moved or
+      // resized while the extended desktop is disabled.
+      // crbug.com/152003.
+      if (base::chromeos::IsRunningOnChromeOS()) {
+        XSetWindowBackground(xdisplay_,
+                             x_root_window_,
+                             BlackPixel(xdisplay_, DefaultScreen(xdisplay_)));
+        XClearWindow(xdisplay_, x_root_window_);
+        // Set the background back to None to preserve content.
+        XSetWindowBackgroundPixmap(xdisplay_, x_root_window_, None);

[Daniel Erat, 2012/09/25 19:10:57]

it's not obvious to me why this is necessary.  can we just set the root's
background to black at startup when we're running on chrome os and be done with
it?

[oshima, 2012/09/25 19:33:28]

According to @marcheu, we intentionally set background pixmap to None, to
to avoid flashing when chrome switching to guest mode, or restarting
chrome due to crash.

[Daniel Erat, 2012/09/25 19:54:41]

Got it, thanks.  Wonder if it'd be cleaner to just use XFillRectangle() here. 
The downside is that you might need to create a GC first.  Yay X.

[oshima, 2012/09/25 20:45:09]

Done.

+      }
       break;
     }
     case GenericEvent: