blink/canvas2d: Add null-checks for SkImage in SkiaTextureHolder.

Skia may fail to create an SkImage when initializing the
SkiaTextureHolder from a mailbox or a provided texture id. Handle this
safely in SkiaTextureHolder.

R=fserb@chromium.org

Bug: 978659
Change-Id: I5e76292b55302cd9045568dd4bc912d1d765af98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679737
Auto-Submit: Khushal <khushalsagar@chromium.org>
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673431}

third_party/blink/renderer/platform/graphics/skia_texture_holder.cc

@@ -142,7 +142,7 @@ SkiaTextureHolder::~SkiaTextureHolder() {
 }
 
 bool SkiaTextureHolder::IsValid() const {
-  return !!ContextProviderWrapper();
+  return !!image_ && !!ContextProviderWrapper();
 }
 
 }  // namespace blink

third_party/blink/renderer/platform/graphics/skia_texture_holder.h

@@ -21,10 +21,14 @@ class PLATFORM_EXPORT SkiaTextureHolder final : public TextureHolder {
 
   // TextureHolder impl.
   IntSize Size() const final {
-    return IntSize(image_->width(), image_->height());
+    if (image_)
+      return IntSize(image_->width(), image_->height());
+    return IntSize();
   }
   bool IsValid() const final;
-  bool CurrentFrameKnownToBeOpaque() const final { return image_->isOpaque(); }
+  bool CurrentFrameKnownToBeOpaque() const final {
+    return image_ && image_->isOpaque();
+  }
 
   const sk_sp<SkImage>& GetSkImage() const { return image_; }