Oilpan: fix PromiseTracker's handling of its persistent promise wrappers.

Source/core/inspector/InspectorDebuggerAgent.h

Index: Source/core/inspector/InspectorDebuggerAgent.h
diff --git a/Source/core/inspector/InspectorDebuggerAgent.h b/Source/core/inspector/InspectorDebuggerAgent.h
index 1358a8482719a97e971ac6f7fdba8cf014422e9b..5a7bc7190f305867ddbd1ebc373b526693151fb4 100644
--- a/Source/core/inspector/InspectorDebuggerAgent.h
+++ b/Source/core/inspector/InspectorDebuggerAgent.h
@@ -244,7 +244,8 @@ private:
     String sourceMapURLForScript(const Script&, CompileResult);
 
     PassRefPtrWillBeRawPtr<JavaScriptCallFrame> topCallFrameSkipUnknownSources(String* scriptURL, bool* isBlackboxed);
-    AsyncCallStackTracker& asyncCallStackTracker() { return *m_asyncCallStackTracker; };
+    AsyncCallStackTracker& asyncCallStackTracker() const { return *m_asyncCallStackTracker; };
+    PromiseTracker& promiseTracker() const { return *m_promiseTracker; }
 
     typedef HashMap<String, Script> ScriptsMap;
     typedef HashMap<String, Vector<String> > BreakpointIdToDebugServerBreakpointIdsMap;
@@ -272,7 +273,7 @@ private:
     bool m_skipContentScripts;
     OwnPtr<ScriptRegexp> m_cachedSkipStackRegExp;
     OwnPtrWillBeMember<AsyncCallStackTracker> m_asyncCallStackTracker;
-    PromiseTracker m_promiseTracker;
+    OwnPtrWillBeMember<PromiseTracker> m_promiseTracker;
 };
 
 } // namespace blink