Fix for [CEF 2171] Crash on quit(https://github.com/adobe/brackets/issue...

This is to fix the crash on quit(Issue#7683) after
integrating CEF 2171.
On Destruction, callback_map_ was getting destroyed
in the ClientApp::~ClientApp(). However while removing
all the elements, it was trying to destroy some stale
objects which were already deleted. So to fix this, we
are now explicitly clearing the map in ClientApp::OnContextReleased()
which gets triggered by CEF, just before releasing V8 context
@RaymondLim . I think before deleting an object pointed out by the iterator, it should be incremented so that iterator is pointing to a valid object. callback_map_.erase(it++) is going to increment 'it' iterator, but returns the old value, which then will be passed to erase() function. Another way of this is 

CallbakMap::iterator oldVal = it;
++it;
callback_map_erase(oldVal);

Do you think the second method is better?