[SkCapture] Insert recording capture for uninserted draw tasks

Update SkCaptureManager::captureUninsertedDrawTasks() to pass snapped
pictures to onInsertRecording() so that uninserted draws at capture stop
create a RecordingCapture entry.

Also guard against a null surface in
snapAndIncrement().

Change-Id: I4ddffc995e6bbfb3d48dc4b2965a11f3e02f3568
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1383777
Reviewed-by: Sarah Rust <sarahrust@google.com>
Commit-Queue: Alexis Cruz-Ayala <alexisdavidc@google.com>
Auto-Submit: Alexis Cruz-Ayala <alexisdavidc@google.com>