debugger: Update the picture view when the first command is selected

Update the picture view in the debugger when the first command is
selected. DebugCanvas::drawTo(index) draws up to and including the index.

Review URL: https://codereview.chromium.org/837483002
diff --git a/debugger/SkDebugger.h b/debugger/SkDebugger.h
index 683b98d..5906acc 100644
--- a/debugger/SkDebugger.h
+++ b/debugger/SkDebugger.h
@@ -26,7 +26,7 @@
         fIndex = index;
     }
     void draw(SkCanvas* canvas) {
-        if (fIndex > 0) {
+        if (fIndex >= 0) {
             fDebugCanvas->drawTo(canvas, fIndex);
         }
     }