Exiting from Screen zoom by pressing "Escape" or "Backspace" key.

Exiting from Screen zoom by pressing "Escape" or "Backspace" key.

[Issue]    https://bugs.tizen.org/jira/browse/TT-225
[Problem]  User cannot exit the screen zoom.
[Cause]    Screen zoom is not in ViewManager stack, so it can't be supported
           by onBackPressed callback.
[Solution] Added boost signal in PlatformInputManager on "Escape"
           and "Backspace" keys.
[Verify]   Launch browser > Open web page > More Menu > Screen zoom
           Zoom up/down the page. Then press "escape" -> Screen zoom should
           disappear and the page should return to normal scale.

Change-Id: Ica7a37a9fe15f3bb27bf4f54dce53238b90cb7b5