PasswordStore: Add lots of DCHECKs that we're on the expected task runner

This makes it clear to the casual reader (like me) which things run on
the main thread and which on the background thread.

Bug: none
Change-Id: Ica9eb3f0fc26e0080d5e7c07750e89f51b2192b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1627032
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662589}

components/password_manager/core/browser/password_store.h

@@ -304,8 +304,6 @@ class PasswordStore : protected PasswordStoreSync,
  protected:
   friend class base::RefCountedThreadSafe<PasswordStore>;
 
-  typedef base::Callback<PasswordStoreChangeList(void)> ModificationTask;
-
 // TODO(crbug.com/706392): Fix password reuse detection for Android.
 #if defined(SYNC_PASSWORD_REUSE_DETECTION_ENABLED)
   // Represents a single CheckReuse() request. Implements functionality to