Download webstore CRXs into separate directory.

Currently CRXs downloaded from the webstore (i.e. during extension/app
install) go to the default download directory, but are deleted once
installed. This is particularly obvious on Mac where the downloads
folder will bounce in the dock to indicate a new file.

This CL adds a field trial to make all CRXs downloaded from the webstore
go into their own directory under the user data dir. It also adds UMA for
successful and failed installs from the webstore.

BUG=309536,313992

Review URL: https://codereview.chromium.org/46583007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236167 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236167