First pass at font cache refactor: Create an atlas manager per texture

First pass at font cache refactor: Create an atlas manager per texture

This changes the AtlasMgr from a singleton class to one that is
created per-texture. This is the first step in allowing us to create
Atlases of other types (e.g., combine small icons into one big texture).

R=bsalomon@google.com

Author: jvanverth@google.com

Review URL: https://chromiumcodereview.appspot.com/24608002

git-svn-id: http://skia.googlecode.com/svn/trunk@11468 2bbb7eff-a529-9590-31e7-b0007b416f81