Add missing default return values to slience Wreturn-type

This patch adds two missing default return values to slience the
compilation warning "Wreturn-type" when building dawn.node.

Bug: dawn:1761
Change-Id: I87c8566cabba4c2e14174bf9f38f53ed3c0fdb74
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/128103
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/dawn/native/Buffer.cpp

@@ -251,6 +251,9 @@ wgpu::BufferMapState BufferBase::APIGetMapState() const {
         case BufferState::Unmapped:
         case BufferState::Destroyed:
             return wgpu::BufferMapState::Unmapped;
+        default:
+            UNREACHABLE();
+            return wgpu::BufferMapState::Unmapped;
     }
 }
 

src/dawn/native/Pipeline.cpp

@@ -164,6 +164,9 @@ WGPUCreatePipelineAsyncStatus CreatePipelineAsyncStatusFromErrorType(InternalErr
         case InternalErrorType::Internal:
         case InternalErrorType::OutOfMemory:
             return WGPUCreatePipelineAsyncStatus_InternalError;
+        default:
+            UNREACHABLE();
+            return WGPUCreatePipelineAsyncStatus_Unknown;
     }
 }