Check if the theme supplier is an extension.

Check if the theme supplier is an extension.

On linux aura, a supervised user may get the native theme
as default theme, which is also represented by a
CustomThemeSupplier. When checking if a custom theme is already
installed we therefore need to check if it is an extension type.

BUG=265387
TEST=Create a supervised user on linux aura and check
     if it gets the managed user theme.
TBR=pkotwicz@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=217046

[Adrian Kuegel, 2013-08-12 08:58:46]

Hi Peter,
I hope it is ok that I TBR you on this change. I forgot in my other CL that
checking if there is a theme_supplier_ is not enough, because it could be the
native X11 theme for linux aura. Currently I could have checked for the X11
theme, since it is the only exception, but I think it is safer to check if the
managed user theme is already used or if an extension is installed in case some
other default system theme appears.

[commit-bot: I haz the power, 2013-08-12 08:59:30]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/akuegel@chromium.org/22796002/3001

[commit-bot: I haz the power, 2013-08-12 19:07:13]

Change committed as 217046