Fix member variable naming

This renaming was missed in (MultiPictureDraw is taskgroup aware - https://codereview.chromium.org/684923002)

TBR=reed@google.com

Review URL: https://codereview.chromium.org/654293005

src/core/SkMultiPictureDraw.cpp

@@ -114,7 +114,7 @@ void SkMultiPictureDraw::draw() {
     for (int i = 0; i < count; ++i) {
         const DrawData& data = fGPUDrawData[i];
         // we only expect 1 context for all the canvases
-        SkASSERT(data.canvas->getGrContext() == context);
+        SkASSERT(data.fCanvas->getGrContext() == context);
 
         if (!data.fPaint && data.fMatrix.isIdentity()) {
             // TODO: this path always tries to optimize pictures. Should we