[Tizen] Fix a crash in NativeAppWindowTizen::~NativeAppWindowTizen()

Observer must be removed in OnWindowDestroying and not in the destructor,
as the aura::Window is already destroyed when the NativeAppWindowTizen
destructor runs.

BUG=https://crosswalk-project.org/jira/browse/XWALK-782
Merge pull request #1511 from nagineni/XWALK-782-2

[Tizen] Fix a crash in NativeAppWindowTizen::~NativeAppWindowTizen()