Extract extension-related renderer setup from ExtensionService

Move it into a new class RendererStartupHelper. This allows app_shell and
other clients of the src/extensions module use it without running code in
src/chrome.

Also ensure extension processes are added to the extensions::ProcessMap in
app_shell.

BUG=334710
TEST=Existing extensions browser_tests. Also, running app_shell manually still loads an extension.

Review URL: https://codereview.chromium.org/145003006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247379 0039d316-1c4b-4281-b951-d872f2087c98
Make ServiceWorkerVersion::{Start,Stop}Worker take callback

Also adds content_browsertests for start & stop flow

BUG=313530
TEST=content_browsertests:ServiceWorkerBrowserTest.\*

Review URL: https://codereview.chromium.org/145033012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247380 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247380