Rename apps::AppShell* to apps::Shell*

Rename apps::AppShell* to apps::Shell*

The names are getting long and unwieldy and everything is in the apps
namespace anyway. This is consistent with the ash_shell and content_shell
naming conventions.

BUG=None
TEST=compiles
R=miket@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=240742

[James Cook, 2013-12-13 19:44:34]

miket, PTAL.  Dan and I talked about this - we think shorter file and class
names are slightly better, and it's easier to change now than later when we have
more classes.

derat, just FYI

[miket_OOO, 2013-12-13 20:31:42]

Concept LGTM. Actual changes rubber-stamp LGTM.

[James Cook, 2013-12-13 20:52:31]

Committed patchset #1 manually as r240742 (presubmit successful).