[ContextEGL] Restore default FBO ID in onUnmakeCurrent() When making an external context current, ContextEGL sets the default FBO ID to that bound by the external context. However, we don't currently restore it to its previous value (expected to be 0) in onUnmakeCurrent(). Aside from being generally problematic, this specifically causes an assert to go off when later destroying this framebuffer (see the linked bug). This CL adds this restoration. This should be sufficient to resolve the linked bug, as we don't expect ANGLE to process destruction of an external context while it is current due to the GL spec [1]. [1] https://bugs.chromium.org/p/chromium/issues/detail?id=1434315#c6 Bug: chromium:1434315 Change-Id: I6da1880aea4c5245f1506ccc22f92370babd1917 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4451908 Commit-Queue: Geoff Lang <geofflang@chromium.org> Auto-Submit: Colin Blundell <blundell@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>