ProcessStatsDataSource test

Tests that ProcessStatsDataSource writes the process data
into the trace writer as requested.
This CL includes a trace writer to be used for testing that allows the test to
read what was written.

Change-Id: Iaf78aac5fef9481594a17f7a667e860fb9297c62
diff --git a/Android.bp b/Android.bp
index 91d8a96..2406d00 100644
--- a/Android.bp
+++ b/Android.bp
@@ -3680,6 +3680,7 @@
     "src/tracing/core/trace_config.cc",
     "src/tracing/core/trace_packet.cc",
     "src/tracing/core/trace_packet_unittest.cc",
+    "src/tracing/core/trace_writer_for_testing.cc",
     "src/tracing/core/trace_writer_impl.cc",
     "src/tracing/core/trace_writer_impl_unittest.cc",
     "src/tracing/core/virtual_destructors.cc",