Do not clear the source device in SkGpuDevice::drawDevice

Do not clear the source device in SkGpuDevice::drawDevice based on
the status whether the destination device needs a clear.

Source device clear is flushed in SkGpuDevice::accessRenderTarget()
and target device clear is flushed in CHECK_SHOULD_DRAW(draw).

Review URL: https://codereview.chromium.org/943623002
1 file changed