Align validation in `dom::webgpu::Buffer::GetMappedRange` to
correspond with that given in the WebGPU specification. Change
variables to more closely resemble the terms used in the
specification.
Change `dom::webgpu::Buffer::mArrayBuffers` to an `nsTArray` of a new
struct `MappedView`, which carries enough information to properly
validate requests for mapped ranges. Update all uses. Provide an
`nsTArray_RelocationStrategy` specialization for `MappedView`.
Add a new mochitest, `test_buffer_mapping_overlapping_views.html`.
Differential Revision: https://phabricator.services.mozilla.com/D215442
e792d2adff