Change where layer hoisting data is gathered

This CL:
1) removes the EXPERIMENTAL_optimize on SkCanvas & SkDevice
2) moves the saveLayer gathering step to endRecording
3) Replaces GPUOptimize with SkRecordComputeLayers
4) Update bench_pictures & render_pictures to provide the new flag

#2 also necessitated moving the BBH computation (and record optimization) out of SkPicture's ctor (and into endRecording)

Review URL: https://codereview.chromium.org/718443002
initial checkin for experimenting

BUG=skia:

Review URL: https://codereview.chromium.org/696063005