Remove gpu-side clip mask merging from clip mask manager

Remove gpu-side clip mask merging from clip mask manager

In the clip mask merging path, the CMM creates new renderTargets and draws to them. In the non-MDB world this is okay b.c. all the draws land in the same drawTarget anyway. In the MDB world the draws for the new renderTargets have to land in different drawTargets.

This can be resolved by a lot of plumbing and refactoring to create drawContexts for the created renderTargets or by removing the mask-merging drawing path.

Since, https://codereview.chromium.org/1424853002/ (Disable gpu-side clip mask merging in the clip mask manager), appears to have stuck, this CL removes the clip mask merging code.

BUG=skia:4094
BUG=skia:4519

Review URL: https://codereview.chromium.org/1418073005