More code clean-ups after creating DeviceMock with Adapter

This patch does more code clean-ups in Device.cpp after we update
the creation of DeviceMock with Adapter:
1. Remove the `virtual` qualifier on the member functions
   `GetInstance()` and `GetPlatform()` of class `DeviceBase` after
   the removal of these overloads in the Mock tests.
2. Remove the check on the nullability of `mAdapter` in `DeviceBase`.

Bug: chromium:42240655
Test: dawn_unittests
Change-Id: I11ba819d2724ed32300131fbdc773299e465c72e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/202497
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Loko Kung <lokokung@google.com>

src/dawn/native/Device.cpp

diff --git a/src/dawn/native/Device.cpp b/src/dawn/native/Device.cpp
index 84e96ed..e58623f 100644
--- a/src/dawn/native/Device.cpp
+++ b/src/dawn/native/Device.cpp
@@ -1869,12 +1869,8 @@
     }
 
     // Language features are enabled instance-wide.
-    // mAdapter is not set for mock test devices.
-    // TODO(crbug.com/dawn/1702): using a mock adapter and instance could avoid the null checking.
-    if (mAdapter != nullptr) {
-        const auto& allowedFeatures = GetInstance()->GetAllowedWGSLLanguageFeatures();
-        mWGSLAllowedFeatures.features = {allowedFeatures.begin(), allowedFeatures.end()};
-    }
+    const auto& allowedFeatures = GetInstance()->GetAllowedWGSLLanguageFeatures();
+    mWGSLAllowedFeatures.features = {allowedFeatures.begin(), allowedFeatures.end()};
 }
 
 const tint::wgsl::AllowedFeatures& DeviceBase::GetWGSLAllowedFeatures() const {

src/dawn/native/Device.h

diff --git a/src/dawn/native/Device.h b/src/dawn/native/Device.h
index cea3471..3dc04ec 100644
--- a/src/dawn/native/Device.h
+++ b/src/dawn/native/Device.h
@@ -127,12 +127,10 @@
 
     MaybeError ValidateObject(const ApiObjectBase* object) const;
 
-    // TODO(dawn:1702) Remove virtual when we mock the adapter.
-    virtual InstanceBase* GetInstance() const;
-
+    InstanceBase* GetInstance() const;
     AdapterBase* GetAdapter() const;
     PhysicalDeviceBase* GetPhysicalDevice() const;
-    virtual dawn::platform::Platform* GetPlatform() const;
+    dawn::platform::Platform* GetPlatform() const;
 
     // Returns the Format corresponding to the wgpu::TextureFormat or an error if the format
     // isn't a valid wgpu::TextureFormat or isn't supported by this device.