Convert extensions::ProcessMap to BrowserContextKeyedService

This allows us to remove more ExtensionService dependencies, which we need
to do because app_shell doesn't have an ExtensionService.

BUG=334706
TEST=existing unit_tests, browser_tests
TBR=sky@chromium.org for mechanical changes across the non-extensions bits of chrome/browser/

Review URL: https://codereview.chromium.org/139943005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245556 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245556