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

src/gpu/GrInOrderDrawBuffer.cpp

Index: src/gpu/GrInOrderDrawBuffer.cpp
diff --git a/src/gpu/GrInOrderDrawBuffer.cpp b/src/gpu/GrInOrderDrawBuffer.cpp
index b671742309f862783f77b07a53b18795305f36cb..04ce17b8b6d2f55ed3291144493dbc4c76fdb37d 100644
--- a/src/gpu/GrInOrderDrawBuffer.cpp
+++ b/src/gpu/GrInOrderDrawBuffer.cpp
@@ -450,13 +450,10 @@ void GrInOrderDrawBuffer::clearStencilClip(const SkIRect& rect,
 }
 
 void GrInOrderDrawBuffer::discard(GrRenderTarget* renderTarget) {
+    SkASSERT(renderTarget);
     if (!this->caps()->discardRenderTargetSupport()) {
         return;
     }
-    if (NULL == renderTarget) {
-        renderTarget = this->drawState()->getRenderTarget();
-        SkASSERT(renderTarget);
-    }
     Clear* clr = GrNEW_APPEND_TO_RECORDER(fCmdBuffer, Clear, (renderTarget));
     clr->fColor = GrColor_ILLEGAL;
     this->recordTraceMarkersIfNecessary();