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

Review URL: https://codereview.chromium.org/680413005
diff --git a/src/gpu/GrGpu.cpp b/src/gpu/GrGpu.cpp
index 2f4f0a6..f7b9537 100644
--- a/src/gpu/GrGpu.cpp
+++ b/src/gpu/GrGpu.cpp
@@ -198,13 +198,7 @@
                   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);
 }