baseVertex in drawIndexed() should be int32, not uint32

baseVertex in drawIndexed() is int32 in web idl. But it is uint32 in Dawn
project. I also checked the native Graphics API sets. baseVertex is int32
on all native APIs (D3D12, Vulkan, Metal). So this feature in web idl is
definitely correct.

This small change makes dawn match web idl for this feature. And it also
adds a couple of tests to cover negative baseVertex.

BUG=dawn:135
TEST=dawn_end2end_tests

Change-Id: I642c96faed9103c8392979f3714527ed0aac4089
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/6442
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/dawn.json b/dawn.json
index 7528d46..d1c321f 100644
--- a/dawn.json
+++ b/dawn.json
@@ -776,7 +776,7 @@
                     {"name": "index count", "type": "uint32_t"},
                     {"name": "instance count", "type": "uint32_t"},
                     {"name": "first index", "type": "uint32_t"},
-                    {"name": "base vertex", "type": "uint32_t"},
+                    {"name": "base vertex", "type": "int32_t"},
                     {"name": "first instance", "type": "uint32_t"}
                 ]
             },