Previously the SwapLayer was kept in the CALayer hierarchy after
WindowSurfaceCGL's destruction which caused SwapLayer's callbacks to be
called after the shared data was freed, causing a segfault.

BUG=angleproject:1233

Change-Id: I71dce45330e6b7d5f06a41198c10890b1d8d520e
Reviewed-on: https://chromium-review.googlesource.com/320711
Tryjob-Request: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>