Reshuffle include/ directory layout

This CL reorganizes the include/ directory as follows:
1. The immediate subdirectories of include/ are now the stable API
   surface exposed to all embedders, with the exception of ext/.
2. ext/ contains the unstable API surface that is exposed only to chrome.

Test: builds on all configurations and passes tests
Bug: 132678367
Change-Id: If638d4b4c150fd591cd1db851283b25ff6083ed6

src/tracing/data_source.cc

diff --git a/src/tracing/data_source.cc b/src/tracing/data_source.cc
new file mode 100644
index 0000000..edb00fd
--- /dev/null
+++ b/src/tracing/data_source.cc
@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "perfetto/tracing/data_source.h"
+
+namespace perfetto {
+
+DataSourceBase::~DataSourceBase() = default;
+void DataSourceBase::OnSetup(const SetupArgs&) {}
+void DataSourceBase::OnStart(const StartArgs&) {}
+void DataSourceBase::OnStop(const StopArgs&) {}
+
+}  // namespace perfetto