Rename variables in SkCanvas::internalSaveLayer

This is split out of https://codereview.chromium.org/2145343005/ (Recast draw_filter_into_device as drawDevice) to, hopefully, make it easier to integrate that CL

TBR=reed@google.com
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Review-Url: https://codereview.chromium.org/2162883002
1 file changed