Merge 21428 - Allow two user level installs of Chrome to have default browser...

Merge 21428 - Allow two user level installs of Chrome to have default browser settings.

This change will allow two Windows users to install thier own copy of Chrome
and set it as default browser. There is a bunch of refactoring in this change.
 Divided the registry entries into three categories:
  * Entries that are only read from HKLM
  * Entries that actually tie file/protocol associations to Chrome. This will
    almost always be in HKCU (except when it is a OEM install).
  * Entries that we can create in HKCU if we fail to create them in HKLM.
 For single user machine nothing should change.
 For two users on the same machine (each with a different user level Chrome
installed), the second user's default browser will get renamed to
Chromium.<user> and the required registry entries will also be renamed so that
they do not overwrite first users's registry entries.

BUG=3641
TEST=Install and set Chromium as default browser from two different logins on the same machine for various combinations of XP/Vista, admin users/non admin user, userlevel install/systemlevel install.

Review URL: http://codereview.chromium.org/159172

TBR=kuchhal@chromium.org


Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=23192