GCC 4.6 fix: intptr_t <-> NULL triggers an warning.

BUG=chromium:87001
TEST=none

Patch by Ryan Sleevi <rsleevi@chromium.org>
Review URL: http://codereview.appspot.com/4571048/

git-svn-id: http://skia.googlecode.com/svn/trunk@1538 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/src/GrContext.cpp b/gpu/src/GrContext.cpp
index 00ca69a..8baa855 100644
--- a/gpu/src/GrContext.cpp
+++ b/gpu/src/GrContext.cpp
@@ -57,7 +57,7 @@
 }
 
 GrContext* GrContext::CreateGLShaderContext() {
-    return GrContext::Create(kOpenGL_Shaders_GrEngine, NULL);
+    return GrContext::Create(kOpenGL_Shaders_GrEngine, 0);
 }
 
 GrContext::~GrContext() {