Elim Shell:browser_context

This is some cleanup from another CL where I discovered that
Shell::Get()->browser_context() isn't actually used by Chrome, only by
ash_shell. To eliminate that confusion I eliminated it and changed the
ash_shell code to rely on delegate()->GetCurrentBrowserContext() to be
consistent with Chrome.

BUG=none
R=ben@chromium.org, jamescook@chromium.org

Review URL: https://codereview.chromium.org/59153009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234024 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234024