Fix Kokoro build

Change-Id: I6f6f643c4f793e0af846a581f37576bca2caea9d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51140
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>

src/sem/variable.h

diff --git a/src/sem/variable.h b/src/sem/variable.h
index 49f2340..2b2b2e7 100644
--- a/src/sem/variable.h
+++ b/src/sem/variable.h
@@ -81,7 +81,7 @@
   bool IsPipelineConstant() const { return is_pipeline_constant_; }
 
   /// @returns the pipeline constant ID associated with the variable
-  uint32_t ConstantId() const { return constant_id_; }
+  uint16_t ConstantId() const { return constant_id_; }
 
  private:
   const ast::Variable* const declaration_;