Fix bug in plot locking system

In the new MultiPictureDraw tests a single hoisted layer is reused multiple times. The previous plot locking scheme allowed GrCachedLayer objects to be aggressively deleted prematurely leaving the reusing GrHoistedLayer objects with dangling pointers.

This CL changes the plot locking system to add a pseudo-ref for each GrHoistedLayer.

NOTRY=true

Review URL: https://codereview.chromium.org/640323002
2 files changed