ui: Add a DCHECK that we don't set up ContextFactory more than once.

Doing this is weird behaviour and could cause a leak, we should only
set up a ContextFactory once, and clear it before we set a new one.

TBR=piman@chromuium.org
BUG=270918

Review URL: https://codereview.chromium.org/173503005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252591 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252591
Mojo: Refactor MessageInTransit to not be POD.

Instead, make it possibly own a buffer. (This means that we'll make more
allocations for each message; however, this is generally necessary to allow
different kinds of data to be "attached" to the message, at least without going
through extreme contortions.)

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/174093004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252592 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252592