GitLab

Make the existing RawSharedBuffer an implementation of that. Later,
we'll plumb in a "PlatformSupport" that'll produce these things, but for
now keep the (static) creation methods where they are.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/471773002

Cr-Commit-Position: refs/heads/master@{#289667}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@289667 0039d316-1c4b-4281-b951-d872f2087c98