Fix and re-enable SessionBackendTest.EmptyCommand

Change command string accessor to return c_str rather than direct
array index to string.  This fixes an issue whereby iterator
validators on VS2010 would trip on reference to contents_[0] in
SessionBackendTest.EmptyCommand

BUG=132037

Review URL: https://codereview.chromium.org/25667004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226900 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226900