Vulkan: Clean up the CovertVertexBuffer* API Some minor clean up of the API so that the arguments we passed into these two functions CovertVertexBufferGPU and CovertVertexBufferCPU are symmetrical. This CL also factor out the common logic in these two functions into a helper function CalculateOffsetAndVertexCountForConversion(). Bug: b/357622380 Change-Id: I5339850c99d733dfea5039ba74734290f8326bd3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5804529 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com>