Adds pipeline cache and implementation for Vulkan backend.

- Adds testing for Vulkan pipeline caching.
- Removed redundant VK_NULL_HANDLE and use explicit {} initialization for 0 handles when necessary.
- Adds some const qualifiers where applicable and useful.
- Removes overloaded GetCacheKey (const/non-const) versions and exposed the cache key member directly for modifiers in derived classes.

Bug: dawn:549
Change-Id: I5e8ab9716eebc916b813c9d032f8dc1f3f5261bc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86581
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/dawn/native/Device.cpp

@@ -40,6 +40,7 @@
 #include "dawn/native/Instance.h"
 #include "dawn/native/InternalPipelineStore.h"
 #include "dawn/native/ObjectType_autogen.h"
+#include "dawn/native/PipelineCache.h"
 #include "dawn/native/QuerySet.h"
 #include "dawn/native/Queue.h"
 #include "dawn/native/RenderBundleEncoder.h"
@@ -968,6 +969,10 @@ void DeviceBase::UncacheAttachmentState(AttachmentState* obj) {
     ASSERT(removedCount == 1);
 }
 
+Ref<PipelineCacheBase> DeviceBase::GetOrCreatePipelineCache(const CacheKey& key) {
+    return GetOrCreatePipelineCacheImpl(key);
+}
+
 // Object creation API methods
 
 BindGroupBase* DeviceBase::APICreateBindGroup(const BindGroupDescriptor* descriptor) {
@@ -1377,6 +1382,11 @@ ResultOrError<Ref<CommandEncoder>> DeviceBase::CreateCommandEncoder(
     return CommandEncoder::Create(this, descriptor);
 }
 
+// Overwritten on the backends to return pipeline caches if supported.
+Ref<PipelineCacheBase> DeviceBase::GetOrCreatePipelineCacheImpl(const CacheKey& key) {
+    UNREACHABLE();
+}
+
 MaybeError DeviceBase::CreateComputePipelineAsync(const ComputePipelineDescriptor* descriptor,
                                                   WGPUCreateComputePipelineAsyncCallback callback,
                                                   void* userdata) {