[wasm] Correctly init zone in PipelineData

The graph_zone_ field in PipelineData was initialized to nullptr
instead of the correct zone when running the wasm compilation pipeline.
Now it is set to the actual zone of the graph that is being compiled.

Bug: chromium:844591
Change-Id: I8d9a688a292471a45dedcd01b03244baed60e378
Reviewed-on: https://chromium-review.googlesource.com/1095254Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53638}

src/compiler/pipeline.cc

@@ -149,6 +149,7 @@ class PipelineData {
         zone_stats_(zone_stats),
         pipeline_statistics_(pipeline_statistics),
         graph_zone_scope_(zone_stats_, ZONE_NAME),
+        graph_zone_(graph_zone_scope_.zone()),
         graph_(mcgraph->graph()),
         source_positions_(source_positions),
         node_origins_(node_origins),