Fill the TLS padding extension with zero bytes instead of spaces.

R=agl@chromium.org
BUG=315828
TEST=none

Review URL: https://codereview.chromium.org/99253006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238460 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238460