[turbofan] Use new CodeAssembler::GetJSContextParameter.

src/fast-accessor-assembler.cc

Index: src/fast-accessor-assembler.cc
diff --git a/src/fast-accessor-assembler.cc b/src/fast-accessor-assembler.cc
index e09db746664cdcb716bb641969c91db6cac1797e..435d3a8e38274e86d001b75257afb1d45929b2f2 100644
--- a/src/fast-accessor-assembler.cc
+++ b/src/fast-accessor-assembler.cc
@@ -184,12 +184,7 @@ FastAccessorAssembler::ValueId FastAccessorAssembler::Call(
   CallInterfaceDescriptor descriptor = stub.GetCallInterfaceDescriptor();
   DCHECK_EQ(4, descriptor.GetParameterCount());
   DCHECK_EQ(0, descriptor.GetStackParameterCount());
-  // TODO(vogelheim): There is currently no clean way to retrieve the context
-  //     parameter for a stub and the implementation details are hidden in
-  //     compiler/*. The context_paramter is computed as:
-  //       Linkage::GetJSCallContextParamIndex(descriptor->JSParameterCount())
-  const int kContextParameter = 3;
-  Node* context = assembler_->Parameter(kContextParameter);
+  Node* context = assembler_->GetJSContextParameter();
   Node* target = assembler_->HeapConstant(stub.GetCode());
 
   Node* call = assembler_->CallStub(