Implement WebGPUDecoderImpl::GetLogPrefix

This was hit during tracing with:

  run_web_tests.py --enable-tracing='*,disabled-by-default-gpu.*'

Change-Id: I9f2a66f1f2b8279732c66e37793541a22397bff2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2242197
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777518}

gpu/command_buffer/service/webgpu_decoder_impl.cc

@@ -435,10 +435,7 @@ class WebGPUDecoderImpl final : public WebGPUDecoder {
                           const volatile void* buffer,
                           int num_entries,
                           int* entries_processed) override;
-  base::StringPiece GetLogPrefix() override {
-    NOTIMPLEMENTED();
-    return "";
-  }
+  base::StringPiece GetLogPrefix() override { return "WebGPUDecoderImpl"; }
   void BindImage(uint32_t client_texture_id,
                  uint32_t texture_target,
                  gl::GLImage* image,