GitLab

This change makes each platform specific ChannelImpl into
a subclass of Channel: ChannelPosix, ChannelWin, ChannelNacl.
delegated functions are now virtual.

TEST=none
BUG=377980
R=darin@chromium.org, jam@chromium.org

Review URL: https://codereview.chromium.org/310293002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275505 0039d316-1c4b-4281-b951-d872f2087c98