Add onBackendCreated callback to Viewer

Also removes fWidth and fHeight from Window and instead
calls into WindowContent to get these values.

BUG=skia:

Change-Id: I72ee506004b7da73db9abb607a3bc82edfcf7d43
Reviewed-on: https://skia-review.googlesource.com/8795
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Yuqian Li <liyuqian@google.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/tools/viewer/ImageSlide.cpp b/tools/viewer/ImageSlide.cpp
index dd7adba646..8ff7d180e3 100644
--- a/tools/viewer/ImageSlide.cpp
+++ b/tools/viewer/ImageSlide.cpp
@@ -20,6 +20,7 @@ SkISize ImageSlide::getDimensions() const {
 }
 
 void ImageSlide::draw(SkCanvas* canvas) {
+    SkASSERT(fImage);
     canvas->drawImage(fImage, 0, 0);
 }