Introduce USE_OPENSSL_CERTS for certificate handling.

See discussion at chromium issue 338885.

When USE_OPENSSL_CERTS is defined, X509::OSCertHandle is now
typedef'ed to struct X509*. 
 
When USE_OPENSSL is defined, USE_OPENSSL_CERTS will now be
defined for linux and Android, while being off for Mac and 
Windows. This allows OpenSSL to be used while leaving
certificate handling to the OS.
 
OpenSSL cert verifying code will only be used on Linux.

This patch does not change any default behavior.

Bug=none
Test=none

Review URL: https://codereview.chromium.org/206453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260152 0039d316-1c4b-4281-b951-d872f2087c98
13 files changed