Fix BindGroupLayout caused by caching.

When a BGL was first created in the cache, it started with an external
refcount of 1 and we then referenced again in
BindGroupLayoutBuilder::GetResultImpl, causing a leak.

Fix this by referencing in the cache lookup only if the BGL already
exists.

src/backend/BindGroupLayout.cpp

@@ -109,7 +109,6 @@ namespace backend {
         BindGroupLayoutBase blueprint(this, true);
 
         auto* result = mDevice->GetOrCreateBindGroupLayout(&blueprint, this);
-        result->Reference();
         return result;
     }
 

src/backend/Device.cpp

@@ -82,6 +82,7 @@ namespace backend {
         // modified.
         auto iter = mCaches->bindGroupLayouts.find(const_cast<BindGroupLayoutBase*>(blueprint));
         if (iter != mCaches->bindGroupLayouts.end()) {
+            (*iter)->Reference();
             return *iter;
         }