Use kTopLeft_GrSurfaceOrigin when generating a GPU buffer for CPU readback.

When attempting to read back a buffer from the GPU, Skia will perform a
y-flip of the buffer in CPU memory if the buffer does not have a
top-left origin.

Test: CtsUiRenderingTestCases
Bug: 119366261
Change-Id: I93ec93cc31209cbdd9b886b0d1020fda3aac646e