Android: Remove unused browser compositor method

BUG=258662
NOTRY=True

Review URL: https://codereview.chromium.org/26334004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228550 0039d316-1c4b-4281-b951-d872f2087c98
Fix NULL pointer exception when ServiceDiscoveryClientMdns is shutdown before the discovery thread is initialized.

BUG=307070
TEST=Ran MDnsAPITest.RegisterListener browser_test with ASAN build

Review URL: https://codereview.chromium.org/26687003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228551 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228551