Android WebView: Allow CookieManager to be used without starting Chromium.

Refactor the CookieManager to allow it to be used without starting
Chromium. If Chromium has not been started when the cookie manager is
first used, a cookie monster will be created and used to service
requests (with its own client/backend threads). If the app subsequently
triggers regular Chromium startup (e.g. by creating a WebView) then this
existing cookie manager will be used instead of creating one on the
regular Chromium threads. CookieMonster is threadsafe, so this is safe
(it just uses two additional threads).

If apps don't touch CookieManager until after Chromium has been started
for some other reason, then CookieMonster will be created as normal on
the regular Chromium threads.

BUG=304813

Review URL: https://codereview.chromium.org/71583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237338 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237338