Rename BookmarkModelObserver::Loaded() method to BookmarkModelLoaded().

Every other method in BookmarkModelObserver starts with either
BookmarkModel if it is related to BookmarkModel or or BookmarkNode if it
is related to BookmarkNodes.

To be consistent Loaded() should start with BookmarkModel since it is
saying the model has been loaded.

BUG=330428
TEST=None, no functional changes.
R=brettw@chromium.org
TBR=ben

Review URL: https://codereview.chromium.org/120853002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242515 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242515