Add rewind capability to SkChunkAlloc

Split off from https://codereview.chromium.org/940533003/ (Decrease GrInOrderDrawBuffer::Cmd's reliance on GrInOrderDrawBuffer)

Review URL: https://codereview.chromium.org/967553003
diff --git a/src/utils/SkDeferredCanvas.cpp b/src/utils/SkDeferredCanvas.cpp
index 7da8532..8fe9f8a 100644
--- a/src/utils/SkDeferredCanvas.cpp
+++ b/src/utils/SkDeferredCanvas.cpp
@@ -68,7 +68,7 @@
     void* requestBlock(size_t minRequest, size_t* actual) SK_OVERRIDE;
     void notifyWritten(size_t bytes) SK_OVERRIDE;
     void playback(bool silent);
-    bool hasPendingCommands() const { return fAllocator.blockCount() != 0; }
+    bool hasPendingCommands() const { return fAllocator.totalUsed() != 0; }
     size_t storageAllocatedForRecording() const { return fAllocator.totalCapacity(); }
 private:
     enum {