trace_processor: Add slices from ftrace traces

- Refactor slice tracking code from json_trace_parser
  into slice_tracker.
- Introduce Print parsing code in proto_trace_parser.
- Add tests for slice tracking code.

Change-Id: Iec14860f45e1eb36a884ef1ddd91ba35d2e91132
diff --git a/src/trace_processor/trace_storage.h b/src/trace_processor/trace_storage.h
index 925f307..2fe305d 100644
--- a/src/trace_processor/trace_storage.h
+++ b/src/trace_processor/trace_storage.h
@@ -103,7 +103,7 @@
     const std::deque<uint64_t>& cycles() const { return cycles_; }
 
    private:
-    // Each vector below has the same number of entries (the number of slices
+    // Each deque below has the same number of entries (the number of slices
     // in the trace for the CPU).
     std::deque<uint64_t> start_ns_;
     std::deque<uint64_t> durations_;