Add LoadLog instrumentation to HttpNetworkTransaction, to tell how much time was spent reading/writing headers and reading/writing body.

Review URL: http://codereview.chromium.org/384098

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@31861 0039d316-1c4b-4281-b951-d872f2087c98

• net/base/load_log_event_type_list.h[diff]
• net/http/http_network_transaction.cc[diff]
• net/url_request/url_request_unittest.cc[diff]