Disable gpu-side clip mask merging in the clip mask manager

In the MDB world the clip mask manager would need to create a separate drawContext for each temporary mask (and we would need to support stencil draws in the drawContext). For now, disable the feature.

Please see skbug.com/4519 (Re-enable gpu-side mask merging in Ganesh)

BUG=skia:4094

Review URL: https://codereview.chromium.org/1424853002
2 files changed