Make content shell activate and focus dev tools on show.

Add Activate() function to ShellDevToolsFrontend class, and call Activate() and Focus() on Show().

BUG=

Review URL: https://codereview.chromium.org/31043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229709 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229709