fixed uninitialized value warning in GrGLShaderStringBuilder

Bug: skia:
Change-Id: I2dda92573acc05ecf0e2c2ac34a26c6c26822c0a
Reviewed-on: https://skia-review.googlesource.com/91000
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed