Reenable D3D12 tests that failed with 'tint_first_vertex_index not found'

Fixed: tint:687
Change-Id: I14eb788f9499f38c2bbe8e0df71399c98281c087
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/47424
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tests/end2end/VertexFormatTests.cpp

@@ -49,9 +49,6 @@ class VertexFormatTest : public DawnTest {
     void SetUp() override {
         DawnTest::SetUp();
 
-        // TODO(crbug.com/tint/687): error: struct member tint_first_vertex_index not found
-        DAWN_SKIP_TEST_IF(IsD3D12() && HasToggleEnabled("use_tint_generator"));
-
         renderPass = utils::CreateBasicRenderPass(device, kRTSize, kRTSize);
     }