Make GrGLShaderBuilder responsible for inserting begining of main() function defition and closing brace.

This will allow another change I am working on to insert code at the begining of a shader.

R=robertphillips@google.com
Review URL: https://codereview.appspot.com/6737058

git-svn-id: http://skia.googlecode.com/svn/trunk@6028 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed