transform::VertexPulling - use DataMap for inputs

Migrate this transform to using the transform::DataMap pattern for configuration.

Allows users to fully construct their transforms ahead of time, and pass in the configuration options for each run.

Bug: tint:389
Change-Id: Ie4a8bf80d7b09cfe7bdd4ef01287d994b6b9eb4f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/47626
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>

src/transform/transform.h

diff --git a/src/transform/transform.h b/src/transform/transform.h
index e87d2c5..2c96b27 100644
--- a/src/transform/transform.h
+++ b/src/transform/transform.h
@@ -36,6 +36,10 @@
 
   /// Destructor
   ~Data() override;
+
+  /// Assignment operator
+  /// @returns this Data
+  Data& operator=(const Data&);
 };
 
 /// DataMap is a map of Data unique pointers keyed by the Data's ClassID.