Record zero-attribute vertex buffer when creating render pipeline

Currently Dawn ignores all zero-attribute vertex buffer in the given
pipeline descriptor when creating RenderPipelineBase because
zero-attribute vertex buffer is treated as unused slot, however the spec
doesn't state that zero-attribute vertex buffer should be ignored.

To support zero-attribute vertex buffer, this commit has the following
changes.

1. Add VertexBufferNotUsed enum value to wgpu::VertexStepMode to
   represent unused slots
2. Ignore VertexBufferNotUsed  step mode buffers when creating
   RenderPipelineBase and add tests to check it
3. Record zero-attribute vertex buffers when creating RenderPipelineBase
   and add tests to check it
4. Fix VertexStateTest::LastAllowedVertexBuffer broken by the above
   changes

Temporarily we set the enum value of
wgpu::VertexStepMode::VertexBufferNotUsed to 0 to pass the CTS tests
because currently empty vertex buffer slots step mode can be
zero-initialized. We will make a CL to Blink to explicitly set
wgpu::VertexStepMode::VertexBufferNotUsed for empty slots and change
the enum value to 2.

Bug: dawn:1000
Change-Id: Ibd4ab87f2c922e8e460f2311547f13d58f1d5611
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/89340
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Takahiro <hogehoge@gachapin.jp>