Don't allow renderTarget==NULL to GrContext::clear() and friends.

src/gpu/gl/GrGpuGL.cpp

Index: src/gpu/gl/GrGpuGL.cpp
diff --git a/src/gpu/gl/GrGpuGL.cpp b/src/gpu/gl/GrGpuGL.cpp
index 6c35e9f2d3f888b626e8a1f8eb3375c6408dba80..621bef8f87517e3c18f5066d72d01f46c245783e 100644
--- a/src/gpu/gl/GrGpuGL.cpp
+++ b/src/gpu/gl/GrGpuGL.cpp
@@ -1401,15 +1401,10 @@ void GrGpuGL::onClear(GrRenderTarget* target, const SkIRect* rect, GrColor color
 }
 
 void GrGpuGL::discard(GrRenderTarget* renderTarget) {
+    SkASSERT(renderTarget);
     if (!this->caps()->discardRenderTargetSupport()) {
         return;
     }
-    if (NULL == renderTarget) {
-        renderTarget = this->drawState()->getRenderTarget();
-        if (NULL == renderTarget) {
-            return;
-        }
-    }
 
     GrGLRenderTarget* glRT = static_cast<GrGLRenderTarget*>(renderTarget);
     if (renderTarget->getUniqueID() != fHWBoundRenderTargetUniqueID) {