protobuf: switch to using third_party/protobuf

This change lights up new code, but the code should be identical to
the code in protobuf2, so no functional changes are expected.

BUG=56579
TEST=sync_unit_tests

http://codereview.chromium.org/3598004/show

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@61185 0039d316-1c4b-4281-b951-d872f2087c98
9 files changed