CL:32489 caused a compilation error on Android since it
implicitly deletes BackingMemory's default constructor.

Change-Id: I48d415aed8ad0166d8d2bd61d06d9ad5ba8a565c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32629
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Hernan Liatis <hliatis@google.com>