Remove WebRequestAPI dependency from extensions::EventRouter

Introduce a WebRequestAPI BrowserContextKeyedService to observe EventRouter
for EventListener additions and removals.

This is part of the AppShell extensions refactor and will make it easier to
move extensions::EventRouter into src/extensions.

BUG=316367
TEST=unit test added, existing browser_tests
R=yoz@chromium.org
TBR=davemoore@chromium.org for trivial change to chrome/browser/profiles/chrome_browser_main_extra_parts_profiles.cc

Review URL: https://codereview.chromium.org/73193008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236142 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236142