internal/trace: support parsing of 1.5 traces

1. Parse out version from trace header.
2. Restore handling of 1.5 traces.
3. Restore optional symbolization of traces.
4. Add some canned 1.5 traces for regression testing
   (http benchmark trace, runtime/trace stress traces,
    plus one with broken timestamps).

Change-Id: Idb18a001d03ded8e13c2730eeeb37c5836e31256
Reviewed-on: https://go-review.googlesource.com/21803
Run-TryBot: Dmitry Vyukov <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

misc/nacl/testzip.proto

@@ -109,6 +109,10 @@ go	src=..
 			png
 				testdata
 					+
+		internal
+			trace
+				testdata
+					+
 		io
 			+
 		mime

src/cmd/trace/main.go

@@ -99,7 +99,7 @@ func parseEvents() ([]*trace.Event, error) {
 		defer tracef.Close()
 
 		// Parse and symbolize.
-		events, err := trace.Parse(bufio.NewReader(tracef))
+		events, err := trace.Parse(bufio.NewReader(tracef), programBinary)
 		if err != nil {
 			loader.err = fmt.Errorf("failed to parse trace: %v", err)
 			return