Handle shader construction failures gracefully

If we cannot instantiate a shader (due to e.g. a non-invertible local
matrix), the expectation is to not draw anything.  We're currently
filling with black.

This unblocks https://skia-review.googlesource.com/c/7885/, which
prevents creation of shaders with non-invertible matrices.

R=reed@google.com

Review-Url: https://codereview.chromium.org/2677633004
Cr-Commit-Position: refs/heads/master@{#448018}

third_party/WebKit/Source/core/svg/graphics/SVGImage.cpp

@@ -320,6 +320,12 @@ void SVGImage::drawPatternForContainer(GraphicsContext& context,
   paint.setShader(MakePaintShaderRecord(tilePicture, SkShader::kRepeat_TileMode,
                                         SkShader::kRepeat_TileMode,
                                         &patternTransform, nullptr));
+  // If the shader could not be instantiated (e.g. non-invertible matrix),
+  // draw transparent.
+  // Note: we can't simply bail, because of arbitrary blend mode.
+  if (!paint.getShader())
+    paint.setColor(SK_ColorTRANSPARENT);
+
   paint.setBlendMode(compositeOp);
   paint.setColorFilter(sk_ref_sp(context.getColorFilter()));
   context.drawRect(dstRect, paint);

third_party/WebKit/Source/platform/graphics/Image.cpp

@@ -303,20 +303,24 @@ void Image::drawPattern(GraphicsContext& context,
   const auto tmy = computeTileMode(destRect.y(), destRect.maxY(), adjustedY,
                                    adjustedY + tileSize.height());
 
-  {
-    PaintFlags paint = context.fillPaint();
-    paint.setColor(SK_ColorBLACK);
-    paint.setBlendMode(compositeOp);
-    paint.setFilterQuality(
-        context.computeFilterQuality(this, destRect, normSrcRect));
-    paint.setAntiAlias(context.shouldAntialias());
-    paint.setShader(
-        createPatternShader(image.get(), localMatrix, paint,
-                            FloatSize(repeatSpacing.width() / scale.width(),
-                                      repeatSpacing.height() / scale.height()),
-                            tmx, tmy));
-    context.drawRect(destRect, paint);
-  }
+  PaintFlags paint = context.fillPaint();
+  paint.setColor(SK_ColorBLACK);
+  paint.setBlendMode(compositeOp);
+  paint.setFilterQuality(
+      context.computeFilterQuality(this, destRect, normSrcRect));
+  paint.setAntiAlias(context.shouldAntialias());
+  paint.setShader(
+      createPatternShader(image.get(), localMatrix, paint,
+                          FloatSize(repeatSpacing.width() / scale.width(),
+                                    repeatSpacing.height() / scale.height()),
+                          tmx, tmy));
+  // If the shader could not be instantiated (e.g. non-invertible matrix),
+  // draw transparent.
+  // Note: we can't simply bail, because of arbitrary blend mode.
+  if (!paint.getShader())
+    paint.setColor(SK_ColorTRANSPARENT);
+
+  context.drawRect(destRect, paint);
 
   if (currentFrameIsLazyDecoded())
     PlatformInstrumentation::didDrawLazyPixelRef(imageID);
@@ -339,6 +343,8 @@ bool Image::applyShader(SkPaint& paint,
 
   paint.setShader(image->makeShader(SkShader::kRepeat_TileMode,
                                     SkShader::kRepeat_TileMode, &localMatrix));
+  if (!paint.getShader())
+    return false;
 
   // Animation is normally refreshed in draw() impls, which we don't call when
   // painting via shaders.