Add WebRequest methods for determining "safe" requests

* This is useful for logic that decides what DB (master/slave) to use
  based on the nature of the web request. It could also be used to
  enforce clear read/write distinctions via exceptions if DB_MASTER
  is misused.
* Also fixed two IDEA errors while editing this class.

Bug: T134608
Change-Id: I43f4bc06c19d823d7d1ffd9cee8bbe60563c7f82