[dawn][wire] Check that buffer is mapped in DeserializeDataUpdate.

Previously the target of the WriteHandle for a buffer was set as soon as
the buffer is mapped. Between the time it was first mapped and the time
DeserializeDataUpdate was called (right before Unmap), the buffer could
be implicitly unmapped by a call to Device::Destroy.

 - Instead check for the buffer being mapped directly in
   DeserializeDataUpdate, which remove the need to track a mapWriteState
   on the ObjectData<WGPUBuffer>.
 - Update the change detecting WireTests to account to GetMappedRange
   being done in a different place now for writable buffers.
 - Add a new test that allows injecting WireCmds directly for even more
   precise but even more change detecting tests.
 - Add necessary backdoors to WireClient and WireTest need for the new
   tests.
 - Link dawn::wire statically in dawn_unittests as we now need to use
   some of its internals directly.

Bug: 492139412
Change-Id: Ibe9ab95ae7456c6629434d4978f439ebfe41c4d1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/296817
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>