HQP Refactoring (in Preparation for SQLite Cache)

HQP Refactoring (in Preparation for SQLite Cache) 

1. Move ownership of the InMemoryURLIndex from the InMemoryHistoryBackend to the HistoryBackend where it truly belongs. 
2. Encapsulate the private, persistent data for the InMemoryURLIndex in a new class, URLIndexPrivateData. 
3. Handle (by notification) URL visits, updates and deletes. Refactor use of NOTIFICATION_HISTORY_URLS_DELETED to provide the deleted URLRow so that row ID is available. 
4. Correctly handle the adding and removing of page title words when a URL change is detected. 
5. Move most of the support types, including the new URLIndexPrivateData class, into a new file, in_memory_url_index_types.h. 
6. Replace static class member functions with non-friend, non-class functions for better flexibility. 
7. Move convenience types out from InMemoryURLIndex class up into history namespace. 
8. Rename convenience types to generalize their intent. 
9. Other small cleanups. 

BUG=96731, 92718
TEST=Unit tests updated. 
TBR=atwilson,brettw

Previously reviewed and LG'ed as http://codereview.chromium.org/8120004/.
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=105678