Fixes possible use after free in RootWindow::DispatchHeldEvents

Code needs to handle the case of RootWindow being deleted during
DispatchHeldEvents.

BUG=309768
TEST=covered by test now
R=ben@chromium.org

Review URL: https://codereview.chromium.org/52543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232062 0039d316-1c4b-4281-b951-d872f2087c98
Enable Desktop Capture API in Beta channel

Current plan is to ship Desktop Capture API in M33. This change enables
it in M32 Beta so apps using the API can be developed with Beta
builds of chrome.

BUG=134249

Review URL: https://codereview.chromium.org/52553003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232063 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232063