Fix GaussianEdgeShader GLSL code

Implicit conversion of int constants to float is not always
guaranteed

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2328263004
TBR=herbderby@google.com
NOTRY=true
NOTREECHECKS=true

Review-Url: https://codereview.chromium.org/2328263004
1 file changed