Dawn: fix memory corruption in DawnTestContext.

Move the dawn_native::Instance ownership from DawnTestContextImpl to
DawnTestContext (its parent class), which owns the wgpu::Device.

dawn_native::Instance must outlive any wgpu::Devices created from it.

Bug: skia: 10311
Change-Id: Iedc4ed94f03b61d5e43cd5c93eb68e24bc4474e1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293852
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
2 files changed