Minor cleanup of clip mask manager

Follow up to https://codereview.chromium.org/1418073005/ (Remove gpu-side clip mask merging from clip mask manager).

The path renderer chain is only ever allocated when it is about to be used (so the delayed initialization doesn't buy us anything).

We can now reduce the lifetime of the pipelineBuilder in createAlphaClipMask

Review URL: https://codereview.chromium.org/1416113006
4 files changed