Update RenderTheme and TestExpectations to use the mock theme for both Linux Aura and Linux Gtk.

Note that the NPAPI plugin tests run on the Gtk implementation, but not the Aura implementation. I am including the TestExpectations to skip them all for now (tracked in bug 318978) so that we can flip between Gtk and Aura at any point and not need to worry about getting failing tests.

R=dglazkov@chromium.org
BUG=318980

Review URL: https://codereview.chromium.org/74503002

git-svn-id: svn://svn.chromium.org/blink/trunk@162169 bbb929c8-8fbe-4397-9dbb-9b2b20218538