Swap render target instead of creating a new gpu device for surface copy-on-write

Swap render target instead of creating a new gpu device for surface copy-on-write

Swap render target of the gpu device instead of creating a new gpu
device when making a copy-on-write upon surface modification.

This removes the SkCanvas::setRootDevice which contains problematic code
when trying to increase the use of SkImages internally in Skia.

BUG=skia:3388

Review URL: https://codereview.chromium.org/925343002