Fix inputCoverageIsUsed bug in GrGLProgramDesc

BUG=skia:
R=joshualitt@chromium.org

Author: egdaniel@google.com

Review URL: https://codereview.chromium.org/593803003
diff --git a/src/gpu/gl/GrGLProgramDesc.cpp b/src/gpu/gl/GrGLProgramDesc.cpp
index 5339937..c356a8c 100644
--- a/src/gpu/gl/GrGLProgramDesc.cpp
+++ b/src/gpu/gl/GrGLProgramDesc.cpp
@@ -173,7 +173,7 @@
     coverageStages->reset();
 
     bool inputColorIsUsed = optState.inputColorIsUsed();
-    bool inputCoverageIsUsed = optState.inputColorIsUsed();
+    bool inputCoverageIsUsed = optState.inputCoverageIsUsed();
 
     // The descriptor is used as a cache key. Thus when a field of the
     // descriptor will not affect program generation (because of the attribute