Remove IFX_SystemHandler.

This CL folds IFX_SystemHandler into CFX_SystemHandler. Methods which either
had no implementation, or returned a default value have been removed.

Review URL: https://codereview.chromium.org/1923093002
diff --git a/fpdfsdk/javascript/JS_Object.cpp b/fpdfsdk/javascript/JS_Object.cpp
index d85cc1d..7dc91a7 100644
--- a/fpdfsdk/javascript/JS_Object.cpp
+++ b/fpdfsdk/javascript/JS_Object.cpp
@@ -107,7 +107,7 @@
       m_swJScript(script),
       m_pRuntime(pRuntime),
       m_pApp(pApp) {
-  IFX_SystemHandler* pHandler = m_pApp->GetSysHandler();
+  CFX_SystemHandler* pHandler = m_pApp->GetSysHandler();
   m_nTimerID = pHandler->SetTimer(dwElapse, TimerProc);
   (*GetGlobalTimerMap())[m_nTimerID] = this;
   m_pRuntime->AddObserver(this);
@@ -123,7 +123,7 @@
 void CJS_Timer::KillJSTimer() {
   if (m_nTimerID) {
     if (m_bValid) {
-      IFX_SystemHandler* pHandler = m_pApp->GetSysHandler();
+      CFX_SystemHandler* pHandler = m_pApp->GetSysHandler();
       pHandler->KillTimer(m_nTimerID);
     }
     GetGlobalTimerMap()->erase(m_nTimerID);