Record UMA counts for number of page loads in windows 8 desktop and metro.

The goal of this change is to collect better metrics about whether users
prefer the metro or the desktop browser experience on Windows 8.  With
the most recent Dev channel push, Metro is the default browser experience
when launching from a start menu tile, but users have the option to switch
back to desktop.  So we would like to get some visibility into this.  We
already track metro -> desktop restarts and desktop -> metro restarts, so
this will give us a rough estimate into the percentage of time people spend
in each experience.

BUG=305369

Review URL: https://codereview.chromium.org/26763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229103 0039d316-1c4b-4281-b951-d872f2087c98
Experimental bookmark model based on tags.

This CL introduces a BookmarTagModel class. BookmarTagModel provides
a way to access and manipulate bookmarks in a non-hierarchical way.
BookmarkTagModel view the bookmarks as a flat list, and each one can be
marked with a collection of tags (tags are simply strings).

BookmarkTagModel converts on demand the data from an existing
BookmarkModel to its view of the world by considering all the titles of all
the ancestors as tags. This view is frozen on an individual bookmarks when
the BookmarkTagModel performs a change on the tags of this bookmarks.

The Bookmark's meta info is used for storage.

An observer may be attached to a BookmarkTagModel to observe relevant
events.

BUG=None

Review URL: https://codereview.chromium.org/26894002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229104 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229104