ChromiumEnv::GetChildren() now clearing prior results.

Required as per API documentation in leveldb/env.h

BUG=324924

Review URL: https://codereview.chromium.org/99913005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238780 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238780