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

src/gpu/GrGpu.cpp

Index: src/gpu/GrGpu.cpp
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index 2f4f0a68c873c3546d3b0db68861c20d14de016e..f7b9537ecc8eb7f98e372b73cc69d214488a8cc9 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -198,13 +198,7 @@ void GrGpu::clear(const SkIRect* rect,
                   GrColor color,
                   bool canIgnoreRect,
                   GrRenderTarget* renderTarget) {
-    if (NULL == renderTarget) {
-        renderTarget = this->getDrawState().getRenderTarget();
-    }
-    if (NULL == renderTarget) {
-        SkASSERT(0);
-        return;
-    }
+    SkASSERT(renderTarget);
     this->handleDirtyContext();
     this->onClear(renderTarget, rect, color, canIgnoreRect);
 }