Co-authored-by: Yulong Wang <[email protected]>

### Description
<!-- Describe your changes. -->

Remove unused variable.

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

Fix Android build error:

```
onnxruntime/onnxruntime/core/providers/webgpu/webgpu_context.h:122:12: error: private field 'query_time_base_' is not used [-Werror,-Wunused-private-field]
  122 |   uint64_t query_time_base_;
      |            ^
1 error generated.
```

### Description

This change allows to set EP option "forceCpuNodeNames" in which
includes a list of node names that will always be running on CPU.

Usage example:

```diff
            opt.executionProviders[0] = {
                name: "webgpu",
                validationMode: 'wgpuOnly',
                storageBufferCacheMode: 'bucket',
+               // force node "/model/embed_tokens/Gather" to be put on CPU
+               forceCpuNodeNames: "/model/embed_tokens/Gather"
            };

```

### Description
<!-- Describe your changes. -->



### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

### Description

The MacOS device on CI is an Apple Paravirtual device, which may not
support TimestampQuery. In this PR, If TimestampQuery is not supported,
a warning is printed to not block profiling cpu data.

### Description

introduce this build flag --use_external_dawn to allow specifying Dawn
as an external dependency. When it is specified, `dawn::native` is
excluded from build of onnxruntime, and passing in dawn proc table is
required.

I didn't find a good way to run the test. Currently I need to build with
the following flags:
```sh
C:\code\onnxruntime>build --use_webgpu --use_external_dawn --skip_tests --target onnxruntime_webgpu_external_dawn_test
```

And run the test using:
```sh
# run test that passing dawn proc table. expect session to initialize successfully
C:\code\onnxruntime\build\Windows\Debug\Debug>onnxruntime_webgpu_external_dawn_test.exe

# run test that does not pass proc table. expect exception to throw.
C:\code\onnxruntime\build\Windows\Debug\Debug>onnxruntime_webgpu_external_dawn_test.exe --no_proc_table
```