Remove TracePacket::Decode and protobuf dependencies

Turns out TracePacket::Decode is used only in tests. Tests
don't justify the complexity of using the SlicedInputStream
(which is going to be deleted by an upcoming CL). Instead
just expose a GetRawBytesForTesting() method and use
explicit deserialization in tests.

Bug: 132880619
Bug: 140126865
Change-Id: I7b8e1422b320b76661438fd18c46186fb82e93ce
diff --git a/test/test_helper.cc b/test/test_helper.cc
index e4af816..57e8b10 100644
--- a/test/test_helper.cc
+++ b/test/test_helper.cc
@@ -61,7 +61,8 @@
 void TestHelper::OnTraceData(std::vector<TracePacket> packets, bool has_more) {
   for (auto& encoded_packet : packets) {
     protos::TracePacket packet;
-    PERFETTO_CHECK(encoded_packet.Decode(&packet));
+    PERFETTO_CHECK(
+        packet.ParseFromString(encoded_packet.GetRawBytesForTesting()));
     if (packet.has_clock_snapshot() || packet.has_trace_config() ||
         packet.has_trace_stats() || !packet.synchronization_marker().empty() ||
         packet.has_system_info()) {