[Android] App crashed when invoking navigator.app.exitApp

The XWalkContent was destroyed when mNativeContent set to 0 in destroy() function.
Don't invoke the webContents.stop after that.
Add similar check to other related APIs.

BUG=XWALK-2815
[TIZEN] Add media class property in manifest for audio streams

This change extends the manifest with 'xwalk_media_app_class' property that
can be used to tag audio streams in pulseaudio and Murphy, so that the Murphy
resource policy daemon can dictate when the audio will be paused and resumed
in response to other audio playing.

This depends on chromium-crosswalk commit 52d4347, so roll
chromium-crosswalk (88fc58a -> 52d4347).

BUG=XWALK-2788
Merge pull request #2558 from fujunwei/exit-app-crash

[Android] App crashed when invoking navigator.app.exitApp
Merge pull request #2579 from nagineni/media_role

 [TIZEN] Add media class property in manifest for audio streams
DEPS.xwalk: Roll v8-crosswalk (03d8b6c -> 35e0d97)

35e0d97 Merge pull request #59 from eleskine/master
089ec67 Extend CPU profiler with mapping ticks to source lines
Bump version to 11.39.238.0