Added tabindex to EULA frame.

BUG=275793
TEST=manual tests on Linux ChromeOS build

Review URL: https://codereview.chromium.org/106513003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238987 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238987