Adds filter support for offscreen canvas

third_party/WebKit/Source/core/paint/FilterEffectBuilder.cpp

Index: third_party/WebKit/Source/core/paint/FilterEffectBuilder.cpp
diff --git a/third_party/WebKit/Source/core/paint/FilterEffectBuilder.cpp b/third_party/WebKit/Source/core/paint/FilterEffectBuilder.cpp
index c1fafc8092dbc528dc9aa8931d9ea81d2f6439bf..5fe7c66e3dcc85586849ad2ec533d2cd9491747f 100644
--- a/third_party/WebKit/Source/core/paint/FilterEffectBuilder.cpp
+++ b/third_party/WebKit/Source/core/paint/FilterEffectBuilder.cpp
@@ -157,6 +157,8 @@ bool FilterEffectBuilder::build(Element* element, const FilterOperations& operat
         FilterOperation* filterOperation = operations.operations().at(i).get();
         switch (filterOperation->type()) {
         case FilterOperation::REFERENCE: {
+            if (!element)

[esprehn, 2016/09/12 20:53:12]

In general core/paint is not thread safe, this code would need to be refactored
out, someone will just break it.

+                break;
             Filter* referenceFilter = buildReferenceFilter(toReferenceFilterOperation(*filterOperation), zoomedReferenceBoxSize, fillPaint, strokePaint, *element, previousEffect, zoom);
             if (referenceFilter)
                 effect = referenceFilter->lastEffect();
@@ -321,4 +323,3 @@ Filter* FilterEffectBuilder::buildReferenceFilter(
 }
 
 } // namespace blink
-