Add ability to track all cookie changes by registration.

Currently, the ability to track all cookie changes is implemented by
a delegate to the CookieMonster.  This CL implements a registration
method on the CookieStore which may be used instead of the delegate.  
This makes the usage of the global tracker more general and more 
easily wrapped by Mojo.  It is not currently implemented on
non-CookieMonster CookieStores, but can be when needed on those
platforms.

Bug: 721395
Change-Id: I6b253b3a2caed7f6c58caf81c9de9f7bacc74b96
Reviewed-on: https://chromium-review.googlesource.com/602775
Commit-Queue: Randy Smith <rdsmith@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498533}

• android_webview/browser/net/aw_cookie_store_wrapper.cc[diff]
• android_webview/browser/net/aw_cookie_store_wrapper.h[diff]
• android_webview/browser/net/aw_cookie_store_wrapper_unittest.cc[diff]
• headless/public/util/testing/generic_url_request_mocks.cc[diff]
• headless/public/util/testing/generic_url_request_mocks.h[diff]
• ios/net/cookies/cookie_store_ios.h[diff]
• ios/net/cookies/cookie_store_ios.mm[diff]
• ios/net/cookies/cookie_store_ios_persistent_unittest.mm[diff]
• ios/net/cookies/cookie_store_ios_unittest.mm[diff]
• net/cookies/cookie_monster.cc[diff]
• net/cookies/cookie_monster.h[diff]
• net/cookies/cookie_monster_unittest.cc[diff]
• net/cookies/cookie_store.h[diff]
• net/cookies/cookie_store_test_helpers.cc[diff]
• net/cookies/cookie_store_test_helpers.h[diff]
• net/cookies/cookie_store_unittest.h[diff]