ok, basic tracing support

This doesn't do anything in the default process-per-task mode, because
those child tasks exit using _exit(), which doesn't trigger the event
tracer destructor to flush.

I don't remember exactly why I exit with _exit(), so I'm going to have
to follow up on that.  But written this way as I think I'm at least
initializing the tracing in the right place for each process for the
future.

In threaded (-j -1) and serial (-j 0) modes, everything seems to work
great.

I'm also thinking I might add a tracer like the SkDebugf tracer but
using ok_log(), which handles interlaced logging from concurrent tasks
better than vanilla SkDebugf.

Example:
    ninja -C out ok; and out/ok gm 8888 filter:search=fontmgr_bounds trace -j -1

Change-Id: Ia3cdad930ce65e6fd12fa74f3fb00894e35138d3
Reviewed-on: https://skia-review.googlesource.com/26350
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

tools/ok_vias.cpp

@@ -5,6 +5,7 @@
  * found in the LICENSE file.
  */
 
+#include "SkEventTracingPriv.h"
 #include "SkImage.h"
 #include "SkOSFile.h"
 #include "SkPictureRecorder.h"
@@ -186,3 +187,28 @@ struct Memory : Dst {
     }
 };
 static Register memory{"memory", "print process maximum memory usage", Memory::Create};
+
+static SkOnce init_tracing_once;
+struct Trace : Dst {
+    std::unique_ptr<Dst> target;
+    std::string trace_mode;
+
+    static std::unique_ptr<Dst> Create(Options options, std::unique_ptr<Dst> dst) {
+        Trace via;
+        via.target = std::move(dst);
+        via.trace_mode = options("mode", "trace.json");
+        return move_unique(via);
+    }
+
+    Status draw(Src* src) override {
+        init_tracing_once([&] { initializeEventTracingForTools(trace_mode.c_str()); });
+        return target->draw(src);
+    }
+
+    sk_sp<SkImage> image() override {
+        return target->image();
+    }
+};
+static Register trace {"trace",
+                       "enable tracing in mode=atrace, mode=debugf, or mode=trace.json",
+                       Trace::Create};

tools/trace/SkEventTracingPriv.cpp

@@ -22,12 +22,14 @@ DEFINE_string(trace, "",
               "               trace events to specified file as JSON, for viewing\n"
               "               with chrome://tracing");
 
-void initializeEventTracingForTools() {
-    if (FLAGS_trace.isEmpty()) {
-        return;
+void initializeEventTracingForTools(const char* traceFlag) {
+    if (!traceFlag) {
+        if (FLAGS_trace.isEmpty()) {
+            return;
+        }
+        traceFlag = FLAGS_trace[0];
     }
 
-    const char* traceFlag = FLAGS_trace[0];
     SkEventTracer* eventTracer = nullptr;
     if (0 == strcmp(traceFlag, "atrace")) {
         eventTracer = new SkATrace();

tools/trace/SkEventTracingPriv.h

@@ -11,9 +11,10 @@
 #include "SkMutex.h"
 
 /**
- * Construct and install an SkEventTracer, based on the 'trace' command line argument.
+ * Construct and install an SkEventTracer, based on the mode,
+ * defaulting to the --trace command line argument.
  */
-void initializeEventTracingForTools();
+void initializeEventTracingForTools(const char* mode = nullptr);
 
 /**
  * Helper class used by internal implementations of SkEventTracer to manage categories.