Check whether bound_instance_ is null in PepperGraphics2DHost::ExecuteTransform

All other accesses of bound_instance_ in this file check if the pointer
is null before using it. This appears to be an oversight.

Bug: 803391
Change-Id: Iaba41bf8e676696a9c7db9bcd327c6125da8c4ed
Reviewed-on: https://chromium-review.googlesource.com/877202Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Raymes Khoury <raymes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533037}

content/renderer/pepper/pepper_graphics_2d_host.cc

@@ -886,9 +886,11 @@ int32_t PepperGraphics2DHost::Flush(PP_Resource* old_image_data) {
 void PepperGraphics2DHost::ExecuteTransform(const float& scale,
                                             const gfx::PointF& translate,
                                             gfx::Rect* invalidated_rect) {
+  if (bound_instance_) {
     bound_instance_->SetGraphics2DTransform(scale, translate);
     *invalidated_rect =
         gfx::Rect(0, 0, image_data_->width(), image_data_->height());
+  }
 }
 
 void PepperGraphics2DHost::ExecutePaintImageData(PPB_ImageData_Impl* image,