Fix SKPBench tiling so MPD and non-MPD match

Fix SKPBench tiling so MPD and non-MPD match

Two issues with the SKPBench tile computation were causing the MPD path to do more work:

The clip from the parent canvas wasn't being used to trim content off the edges of the MPD tiles

The non-MPD path was not taking the scale into account in its tile placement (resulting in it having fewer, larger active tiles when scaling).

Review URL: https://codereview.chromium.org/776273002