Delete background tab IOSurfaces on Mac.

content/browser/renderer_host/compositing_iosurface_mac.mm

Index: content/browser/renderer_host/compositing_iosurface_mac.mm
diff --git a/content/browser/renderer_host/compositing_iosurface_mac.mm b/content/browser/renderer_host/compositing_iosurface_mac.mm
index f1754cf0f97593dca7eaf38fbdc46cc320bb7d3b..1786fb6917d09a6502c297cfc0f05b5c1293294c 100644
--- a/content/browser/renderer_host/compositing_iosurface_mac.mm
+++ b/content/browser/renderer_host/compositing_iosurface_mac.mm
@@ -100,11 +100,13 @@ void CompositingIOSurfaceMac::SetIOSurface(uint64 io_surface_handle) {
 }
 
 void CompositingIOSurfaceMac::DrawIOSurface(NSView* view) {
-  TRACE_EVENT0("browser", "CompositingIOSurfaceMac::DrawIOSurface");
   CGLSetCurrentContext(cglContext_);
 
   bool has_io_surface = MapIOSurfaceToTexture(io_surface_handle_);
 
+  TRACE_EVENT1("browser", "CompositingIOSurfaceMac::DrawIOSurface",
+               "has_io_surface", has_io_surface);
+
   [glContext_ setView:view];
   NSSize window_size = [view frame].size;
   glViewport(0, 0, window_size.width, window_size.height);
@@ -291,6 +293,11 @@ void CompositingIOSurfaceMac::UnrefIOSurfaceWithContextCurrent() {
   }
 
   io_surface_.reset();
+
+  // Forget the ID, because even if it is still around when we want to use it
+  // again, OSX may have reused the same ID for a new tab and we don't want to
+  // blit random tab contents.
+  io_surface_handle_ = 0;
 }
 
 void CompositingIOSurfaceMac::GlobalFrameDidChange() {