nullptr isn't an int; this is what we want

src/gpu/gl/mesa/SkMesaGLContext.cpp

Index: src/gpu/gl/mesa/SkMesaGLContext.cpp
diff --git a/src/gpu/gl/mesa/SkMesaGLContext.cpp b/src/gpu/gl/mesa/SkMesaGLContext.cpp
index 730206e5601975944d0576e31eae398d7484b5e9..541b247fa86cbe8ec66dca6ae1e0876284c64099 100644
--- a/src/gpu/gl/mesa/SkMesaGLContext.cpp
+++ b/src/gpu/gl/mesa/SkMesaGLContext.cpp
@@ -14,7 +14,7 @@
 static const GrGLint gBOGUS_SIZE = 16;
 
 SkMesaGLContext::SkMesaGLContext()
-    : fContext(static_cast<Context>(nullptr))
+    : fContext(static_cast<Context>(0))
     , fImage(nullptr) {
     GR_STATIC_ASSERT(sizeof(Context) == sizeof(OSMesaContext));
 
@@ -79,7 +79,7 @@ void SkMesaGLContext::destroyGLContext() {
 
     if (fContext) {
         OSMesaDestroyContext((OSMesaContext)fContext);
-        fContext = static_cast<Context>(nullptr);
+        fContext = static_cast<Context>(0);
     }
 }