Use a lock to deal with concurrent access to the m_evictedBackings

cc/prioritized_resource.cc

Index: cc/prioritized_resource.cc
diff --git a/cc/prioritized_resource.cc b/cc/prioritized_resource.cc
index ebc8378229575cf4c7e96f68c3f66a7bc0889a58..5f252c041e7bba490f100cd8d939c514b673b0f7 100644
--- a/cc/prioritized_resource.cc
+++ b/cc/prioritized_resource.cc
@@ -107,8 +107,8 @@ void PrioritizedResource::setPixels(ResourceProvider* resourceProvider,
 void PrioritizedResource::link(Backing* backing)
 {
     DCHECK(backing);
-    CHECK(!backing->m_owner);
-    CHECK(!m_backing);
+    DCHECK(!backing->m_owner);
+    DCHECK(!m_backing);
 
     m_backing = backing;
     m_backing->m_owner = this;
@@ -117,7 +117,7 @@ void PrioritizedResource::link(Backing* backing)
 void PrioritizedResource::unlink()
 {
     DCHECK(m_backing);
-    CHECK(m_backing->m_owner == this);
+    DCHECK(m_backing->m_owner == this);
 
     m_backing->m_owner = 0;
     m_backing = 0;
@@ -137,7 +137,6 @@ PrioritizedResource::Backing::Backing(unsigned id, ResourceProvider* resourcePro
     , m_wasAbovePriorityCutoffAtLastPriorityUpdate(false)
     , m_inDrawingImplTree(false)
     , m_resourceHasBeenDeleted(false)
-    , m_inMainThreadEvictedList(false)
 #ifndef NDEBUG
     , m_resourceProvider(resourceProvider)
 #endif
@@ -148,7 +147,6 @@ PrioritizedResource::Backing::~Backing()
 {
     DCHECK(!m_owner);
     DCHECK(m_resourceHasBeenDeleted);
-    CHECK(!m_inMainThreadEvictedList);
 }
 
 void PrioritizedResource::Backing::deleteResource(ResourceProvider* resourceProvider)