QmlProfiler: Avoid nondeterminism when creating trace time object

If start and end times are initialized by clear() the change signals
may or may not be sent.

Change-Id: Ib50a2bbecf58791c5f93348e7450f4c0324f9d90
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/qmlprofiler/qmlprofilermodelmanager.cpp

@@ -86,9 +86,9 @@ void QmlProfilerDataState::setState(QmlProfilerDataState::State state)
 
 
 /////////////////////////////////////////////////////////////////////
-QmlProfilerTraceTime::QmlProfilerTraceTime(QObject *parent) : QObject(parent)
+QmlProfilerTraceTime::QmlProfilerTraceTime(QObject *parent) :
+    QObject(parent), m_startTime(-1), m_endTime(-1)
 {
-    clear();
 }
 
 QmlProfilerTraceTime::~QmlProfilerTraceTime()