Add factory class for generating various flavors of GrTextContext.

Add factory class for generating various flavors of GrTextContext.

This is the first pass of making Gr*TextContext more generic and easily
subclassed. The next stage will be making GrBitmapTextContext and
GrDistanceFieldTextContext more similar by moving the SkDraw loop into
each subclass.

BUG=skia:2018
R=bsalomon@google.com, reed@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/144283002

git-svn-id: http://skia.googlecode.com/svn/trunk@13142 2bbb7eff-a529-9590-31e7-b0007b416f81