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

debugger/SkDebugger.cpp

@@ -11,11 +11,11 @@
 #include "SkString.h"
 
 
-SkDebugger::SkDebugger() {
+SkDebugger::SkDebugger()
+    : fPicture(NULL)
+    , fIndex(-1) {
     // Create this some other dynamic way?
     fDebugCanvas = new SkDebugCanvas(0, 0);
-    fPicture = NULL;
-    fIndex = 0;
 }
 
 SkDebugger::~SkDebugger() {

debugger/SkDebugger.h

@@ -26,7 +26,7 @@ public:
         fIndex = index;
     }
     void draw(SkCanvas* canvas) {
-        if (fIndex > 0) {
+        if (fIndex >= 0) {
             fDebugCanvas->drawTo(canvas, fIndex);
         }
     }