Bug 717698. Add about:jank infrastructure. r=ehsan

Adds a profiling mode that only records samples when we
haven't spun the event loop

tools/profiler/sps/TableTicker.cpp

@@ -245,6 +245,7 @@ class TableTicker: public Sampler {
   {
     mUseStackWalk = hasFeature(aFeatures, aFeatureCount, "stackwalk");
     mProfile.addTag(ProfileEntry('m', "Start"));
+    mJankOnly = hasFeature(aFeatures, aFeatureCount, "jank");
   }
 
   ~TableTicker() { if (IsActive()) Stop(); }
@@ -276,6 +277,7 @@ class TableTicker: public Sampler {
   Stack *mStack;
   bool mSaveRequested;
   bool mUseStackWalk;
+  bool mJankOnly;
 };
 
 /**
@@ -380,6 +382,19 @@ void TableTicker::Tick(TickSample* sample)
   }
   mStack->mQueueClearMarker = true;
 
+  bool recordSample = true;
+  if (mJankOnly) {
+    recordSample = false;
+    // only record the events when we have a we haven't seen a tracer event for 100ms
+    if (!sLastTracerEvent.IsNull()) {
+      TimeDuration delta = sample->timestamp - sLastTracerEvent;
+      if (delta.ToMilliseconds() > 100.0) {
+          recordSample = true;
+      }
+    }
+  }
+
+  if (recordSample) {
 #ifdef USE_BACKTRACE
     if (mUseStackWalk) {
       doBacktrace(mProfile);
@@ -389,8 +404,9 @@ void TableTicker::Tick(TickSample* sample)
 #else
     doSampleStackTrace(mStack, mProfile, sample);
 #endif
+  }
 
-  if (!sLastTracerEvent.IsNull() && sample) {
+  if (!mJankOnly && !sLastTracerEvent.IsNull() && sample) {
     TimeDuration delta = sample->timestamp - sLastTracerEvent;
     mProfile.addTag(ProfileEntry('r', delta.ToMilliseconds()));
   }