[M148][native][d3d12] Fix and rework ShaderVisibleDescriptorAllocator

With https://g-issues.chromium.org/issues/496379067, a bug was
introduced where the first time we pooled a heap, it would pool a
non-max sized one. Also, this CL introduced a performance regression:
because we no longer doubled the heap size when calling
AllocateAndSwitchShaderVisibleHeap, unless we explicitly requested to
allocate a max size heap, most calls would result in allocating a new
heap, and never recycling from a pool.

This CL reworks ShaderVisibleDescriptorAllocator so that we once again
double the heap size when AllocateAndSwitchShaderVisibleHeap is called,
making sure it's large enough to fit the request size.

- Refactored the function to split adding heaps to the pool vs
  retrieving them.
- Added assertions to make sure the heap sizes are what we expect.
- Fixed D3D12DescriptorHeapTests.

Bug: 496379067
Change-Id: I01d5dfb8148d5ed0621901355f2da034f6174fe8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/303915
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
(cherry picked from commit 419c7b7c91a11a1e20196543549bfc8a03dd98ed)
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/305336