[SVGDom] Deferred SampleApp parsing

Parse SVG files in onOnceBeforeDraw() rather than ctor, to avoid
front-loading a bunch of work when passed a loarge number of SVGs.

R=stephana@google.com,robertphillips@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2245993002

Review-Url: https://codereview.chromium.org/2245993002
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 439b699..6297030 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -80,14 +80,14 @@
     }
 };
 
-extern SampleView* CreateSampleSVGFileView(const char filename[]);
+extern SampleView* CreateSampleSVGFileView(const SkString& filename);
 
 class SVGFileFactory : public SkViewFactory {
     SkString fFilename;
 public:
     SVGFileFactory(const SkString& filename) : fFilename(filename) {}
     SkView* operator() () const override {
-        return CreateSampleSVGFileView(fFilename.c_str());
+        return CreateSampleSVGFileView(fFilename);
     }
 };