Replace GrStringBuilder with SkString. First step in cleaning up the shader generator...

Replace GrStringBuilder with SkString. First step in cleaning up the shader generator. Slight performance hit when creating a new shader (<10% of total shader gen time on my Windows box is spent in building our string before handing it to GL). Much of this can be recovered by better usage pattern of SkString in coming revisions.

Review URL: http://codereview.appspot.com/4465053/

git-svn-id: http://skia.googlecode.com/svn/trunk@1266 2bbb7eff-a529-9590-31e7-b0007b416f81