tools/gen_bazel: Standalone Bazel build

Updates the gen_bazel generator and generates
a BUILD file that allows to build traced, traced_probes
perfetto and trace_processor both on Linux and Mac.
This also allows easy integration with google internal builds.
The major differences of the new generator are:
1. Uses the new way of dealing with protobuf libraries
   (proto_library -> cc_proto_library -> executable target
   see go/new-proto-library)
2. Instead of repeating all the source files in each target
   it creates Bazel filegroup(s) out of GN source_set(s).
   While doing so it takes care of propagating up flags and
   dependencies. This is because Bazel filegroup(s),
   conversely to GN source_set(s), are pure bags of sources.

Still missing:
 - cflags / ldflags propagation into Bazel.
 - Android toolchain (perhaps will never happen)

This will require a matching change to Google internal
sources.

Test: bazel build //:all
Change-Id: Ic1c725bd83e2a5e92eebb5c5b34728cf6d5983e6
diff --git a/bazel/jsoncpp.BUILD b/bazel/jsoncpp.BUILD
new file mode 100644
index 0000000..8a33eb8
--- /dev/null
+++ b/bazel/jsoncpp.BUILD
@@ -0,0 +1,49 @@
+# 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.
+
+cc_library(
+    name = "jsoncpp",
+    srcs = [
+        "src/lib_json/json_batchallocator.h",
+        "src/lib_json/json_internalarray.inl",
+        "src/lib_json/json_internalmap.inl",
+        "src/lib_json/json_reader.cpp",
+        "src/lib_json/json_tool.h",
+        "src/lib_json/json_value.cpp",
+        "src/lib_json/json_valueiterator.inl",
+        "src/lib_json/json_writer.cpp",
+    ],
+    hdrs = [
+        "include/json/assertions.h",
+        "include/json/autolink.h",
+        "include/json/config.h",
+        "include/json/features.h",
+        "include/json/forwards.h",
+        "include/json/json.h",
+        "include/json/reader.h",
+        "include/json/value.h",
+        "include/json/version.h",
+        "include/json/writer.h",
+    ],
+    copts = [
+        "-Isrc/lib_json",
+    ],
+    defines = [
+        "JSON_USE_EXCEPTION=0",
+    ],
+    includes = [
+        "include",
+    ],
+    visibility = ["//visibility:public"],
+)