Change samplerVariable() to return a const char *.

This allows backends to use an expression rather than a variable for
sampler access.

Change-Id: I2346ac418c8d6277416a4430e2eaf6ab3ffee0a2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/222036
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
diff --git a/src/gpu/effects/generated/GrMagnifierEffect.cpp b/src/gpu/effects/generated/GrMagnifierEffect.cpp
index 5972aeb..b70c07c 100644
--- a/src/gpu/effects/generated/GrMagnifierEffect.cpp
+++ b/src/gpu/effects/generated/GrMagnifierEffect.cpp
@@ -67,7 +67,7 @@
         fragBuilder->codeAppendf(
                 "d.y), 1.0);\n}\n%s = texture(%s, mix(coord, zoom_coord, weight)).%s;\n",
                 args.fOutputColor,
-                fragBuilder->getProgramBuilder()->samplerVariable(args.fTexSamplers[0]).c_str(),
+                fragBuilder->getProgramBuilder()->samplerVariable(args.fTexSamplers[0]),
                 fragBuilder->getProgramBuilder()->samplerSwizzle(args.fTexSamplers[0]).c_str());
     }