Explicitly initialize the ScatteredStreamWriter used by tracing ProtoWriter

The writer would always end up being initialized in WriteBytes regardless,
but this prevents ClusterFuzz from throwing up false alarms due to
non-zero offsets being applied to a null pointer.

R=eseckler@chromium.org

Bug: 1031563
Change-Id: I3c938bc388194889fa7b7562c30c7d156000fce3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1962982Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724086}

services/tracing/public/cpp/perfetto/traced_value_proto_writer.cc

@@ -53,6 +53,7 @@ class ProtoWriter final : public TracedValue::Writer {
         stream_(&buffer_) {
     proto_.Reset(&stream_);
     buffer_.set_writer(&stream_);
+    stream_.Reset(buffer_.GetNewBuffer());
     node_stack_.emplace(ProtoValueHandle(&proto_));
     proto_.set_nested_type(ProtoValue::DICT);
   }