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 9d08f641eb7d8f4322d7615388fe032710e52ef1..1929ee987b4b070ad254e8a83fd66426ac427409 100644
--- a/src/gpu/gl/GrGpuGL.cpp
+++ b/src/gpu/gl/GrGpuGL.cpp
@@ -1406,15 +1406,10 @@ 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) {