network: Remove an obsolete usage comment in CookieManager

Lily said that it's safe to use CookieManager on the main network sequence, so
I'm removing this obsolete reference to the "IO thread," which scared me into
thinking I'd need to PostTask to use CookieManager accessors.

R=chlily, morlovich

Bug: None
Change-Id: I29b305406db253e10654a096f24ab310220eff1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166604
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Commit-Queue: Maksim Orlovich <morlovich@chromium.org>
Reviewed-by: Lily Chen <chlily@chromium.org>
Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763407}

services/network/cookie_manager.h

@@ -29,10 +29,6 @@ namespace network {
 class SessionCleanupCookieStore;
 
 // Wrap a cookie store in an implementation of the mojo cookie interface.
-
-// This is an IO thread object; all methods on this object must be called on
-// the IO thread.  Note that this does not restrict the locations from which
-// mojo messages may be sent to the object.
 class COMPONENT_EXPORT(NETWORK_SERVICE) CookieManager
     : public mojom::CookieManager {
  public: