Pepper: Clarify use of CHANNEL_DEV.

We permit dev channel APIs to be used on CHANNEL_CANARY, CHANNEL_DEV, and
CHANNEL_UNKNOWN. This has the effect of allowing dev channel interfaces to be
used on Chromium builds.

This patch adds a comment to make this result clearer.

BUG=
R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/117443005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241899 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241899