Merge wxWebView JavaScript improvements branch

This is a squashed commit of the SOC2017_WEBVIEW_JS branch from
https://github.com/joseeloren/wxWidgets.git

Closes https://github.com/wxWidgets/wxWidgets/pull/538

tests/testprec.h

@@ -100,13 +100,13 @@ public:
     #define WX_ASSERT_FAILS_WITH_ASSERT(cond)
 #endif
 
-#define WX_ASSERT_EVENT_OCCURS(eventcounter, count) \
+#define WX_ASSERT_EVENT_OCCURS_IN(eventcounter, count, ms) \
 {\
     wxStopWatch sw; \
     wxEventLoopBase* loop = wxEventLoopBase::GetActive(); \
     while(eventcounter.GetCount() < count) \
     { \
-        if(sw.Time() < 100) \
+        if(sw.Time() < ms) \
             loop->Dispatch(); \
         else \
         { \
@@ -119,6 +119,8 @@ public:
     eventcounter.Clear(); \
 }
 
+#define WX_ASSERT_EVENT_OCCURS(eventcounter,count) WX_ASSERT_EVENT_OCCURS_IN(eventcounter, count, 100)
+
 // these functions can be used to hook into wxApp event processing and are
 // currently used by the events propagation test
 class WXDLLIMPEXP_FWD_BASE wxEvent;