Introduced IDR_SIGNIN_INTERNALS_INDEX_* strings to grit_whitelist.txt

This is to enable chrome://signin-internals

Review URL: https://codereview.chromium.org/70143002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234504 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234504