Prevent re-entrancy in MessagePopupCollection.

Certain call flows would previously cause a use-after-free because of
reentrancy loops between Message Center and the MessagePopupCollection.
This patch rearranges things so that those loops don't happen or cause
problems.

R=dimich@chromium.org
BUG=312856

Review URL: https://codereview.chromium.org/60063003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237440 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237440
Revert 237267 "Enable HTML frame and Transparency by channel, no..."

> Enable HTML frame and Transparency by channel, not experimental flag.
> 
> BUG=322834
> 
> Review URL: https://codereview.chromium.org/84253004

TBR=wez@chromium.org, benwells@chromium.org, garykac@chromium.org

Review URL: https://codereview.chromium.org/89473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237441 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237441