Perfetto: first commit. Introduce build files

This CL sets up the directory structure, build files and
README for Perfetto. The build files are complete enough
to build a hello-world unittest, gtest, gmock and protobuf
from Linux and Mac OSX without depending on any external
tool. This will allow to build the project without a full
Android checkouot and hence being able to get test coverage
on other waterfalls (Chrome, Travis CI) and with a larger
set of compiler toolchains.
The plan is to generate Android.bp build files in the
next CLs to build the same targets also from the Android tree.

Change-Id: I7a15dcfdb24d5f857fc8c3a70757745741b92545
diff --git a/BUILD.gn b/BUILD.gn
new file mode 100644
index 0000000..1eee5b3
--- /dev/null
+++ b/BUILD.gn
@@ -0,0 +1,29 @@
+# Copyright (C) 2017 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.
+
+group("all") {
+  testonly = true  # allow to build also test targets
+  deps = [
+    ":tests",
+    "//buildtools:protobuf_lite",
+  ]
+  deps += [ "//buildtools:protoc($host_toolchain)" ]
+}
+
+group("tests") {
+  testonly = true
+  deps = [
+    "//libtracing:libtracing_unittests",
+  ]
+}