Ensure the callback UpdateNetworkState is not invoked if the object is already deleted

Test case #27 MediaSourceEvents in YouTube HTML5 MSE/EME conformance test
(http://yt-dash-mse-test.commondatastorage.googleapis.com/unit-tests/tot.html) fails
due to a condition where the callback UpdateNetworkState() in WebMediaPlayerAndroid
object is invoked after the object is already deleted. This CL prevents such
case from taking place.

BUG=308861

Review URL: https://codereview.chromium.org/35193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230657 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230657