Mojo: Proper implementations of WriteData/ReadData.

These no longer use the two-phase versions, and thus write/read as much
as possible (not merely as much as possible in a single memcpy).

R=davemoore@chromium.org

Review URL: https://codereview.chromium.org/122583005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242830 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242830