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/tools/gen_merged_sql_metrics.py b/tools/gen_merged_sql_metrics.py
index cb36db5..319d0d3 100755
--- a/tools/gen_merged_sql_metrics.py
+++ b/tools/gen_merged_sql_metrics.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python
 # Copyright (C) 2019 The Android Open Source Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -104,8 +104,7 @@
     for path in sql_outputs.keys():
       name = os.path.basename(path)
       variable = filename_to_variable(os.path.splitext(name)[0])
-      path_escaped = path.encode('string_escape')
-      output.write('\n  {{"{}", {}}},\n'.format(path_escaped, variable))
+      output.write('\n  {{"{}", {}}},\n'.format(path, variable))
     output.write("};\n")
 
     output.write(NAMESPACE_END)