Add histograms separating user- and externally-installed apps and extensions.
Add histograms separating user- and externally-installed apps and extensions.

Remove the LoadExternal histogram that combined externally-installed apps and extensions, since we have each individually now.

BUG=87251
TEST=Verify that LoadApp* and LoadExtension* on chrome://histograms look sensible.

Review URL: http://codereview.chromium.org/7453009

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@93731 0039d316-1c4b-4281-b951-d872f2087c98

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