Improve Mac SSL code:
- Ensure that when OnTransportWriteComplete calls back to SSLWriteCallback, SSLWriteCallback doesn't think that a write is in progress (it _was_, but now it's complete and has to be done again).
- Remove all the "slop" variables; they're not needed now that we have independent IOBuffers to call back to our transport.

BUG=http://crbug.com/21268
TEST=as in bug

Review URL: http://codereview.chromium.org/371008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@31227 0039d316-1c4b-4281-b951-d872f2087c98

• net/socket/ssl_client_socket_mac.cc[diff]
• net/socket/ssl_client_socket_mac.h[diff]