Query API: Non Precise Occlusion Query

- Add BeginOcclusionQuery and EndOcclusionQuery in frontend
- Set occlusion query as unsafe APIs
- Add validation tests

Bug: dawn:434
Change-Id: I3d1cefed780812dd62fb082287ff71530b76ebee
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/31321
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn_native/opengl/CommandBufferGL.cpp

@@ -1221,6 +1221,14 @@ namespace dawn_native { namespace opengl {
                     break;
                 }
 
+                case Command::BeginOcclusionQuery: {
+                    return DAWN_UNIMPLEMENTED_ERROR("BeginOcclusionQuery unimplemented.");
+                }
+
+                case Command::EndOcclusionQuery: {
+                    return DAWN_UNIMPLEMENTED_ERROR("EndOcclusionQuery unimplemented.");
+                }
+
                 case Command::WriteTimestamp:
                     return DAWN_UNIMPLEMENTED_ERROR("WriteTimestamp unimplemented");