add clip to PictureRenderer to better mimic Chromium's rendering method

Since Chromium uses separate canvases when rendering tiles they get an implicit clip. This adds an explicit clip since the PictureRenderer reuses the same canvas.

Review URL: https://codereview.chromium.org/739303005
diff --git a/tools/PictureRenderer.cpp b/tools/PictureRenderer.cpp
index f2fd6b7..33766e9 100644
--- a/tools/PictureRenderer.cpp
+++ b/tools/PictureRenderer.cpp
@@ -618,6 +618,7 @@
     SkMatrix mat(canvas->getTotalMatrix());
     mat.postTranslate(-SkIntToScalar(tileRect.fLeft), -SkIntToScalar(tileRect.fTop));
     canvas->setMatrix(mat);
+    canvas->clipRect(SkRect::Make(tileRect));
     canvas->clear(SK_ColorTRANSPARENT); // Not every picture covers the entirety of every tile
     canvas->drawPicture(picture);
     canvas->restoreToCount(saveCount);