Split extensions::MenuManager instance out from ExtensionService.

This is now its own BCKS. There was no need for it to be in
ExtensionService.

TBR=sky@chromium.org
BUG=162530

Review URL: https://codereview.chromium.org/64953004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235120 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235120