Oilpan: fix build after r355906.

TBR=oilpan-reviews
BUG=475655
NOTRY=true

Review URL: https://codereview.chromium.org/1426553002

Cr-Commit-Position: refs/heads/master@{#355965}
diff --git a/third_party/WebKit/Source/core/dom/Document.cpp b/third_party/WebKit/Source/core/dom/Document.cpp
index 2afcc40..fb85c23 100644
--- a/third_party/WebKit/Source/core/dom/Document.cpp
+++ b/third_party/WebKit/Source/core/dom/Document.cpp
@@ -5744,7 +5744,6 @@
     visitor->trace(m_listsInvalidatedAtDocument);
     for (int i = 0; i < numNodeListInvalidationTypes; ++i)
         visitor->trace(m_nodeLists[i]);
-    visitor->trace(m_cssCanvasElements);
     visitor->trace(m_topLayerElements);
     visitor->trace(m_elemSheet);
     visitor->trace(m_nodeIterators);
diff --git a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
index 77e329b..2f7e0bb 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
@@ -753,10 +753,7 @@
 
 DEFINE_TRACE(HTMLCanvasElement)
 {
-#if ENABLE(OILPAN)
-    visitor->trace(m_observers);
     visitor->trace(m_context);
-#endif
     DocumentVisibilityObserver::trace(visitor);
     HTMLElement::trace(visitor);
 }