[stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

src/arm/macro-assembler-arm.h

Index: src/arm/macro-assembler-arm.h
diff --git a/src/arm/macro-assembler-arm.h b/src/arm/macro-assembler-arm.h
index 9626628877aa8e6ab8126200958c910339a4d8a2..a9c2203520de4ddfcf9cb537cf223393514b0171 100644
--- a/src/arm/macro-assembler-arm.h
+++ b/src/arm/macro-assembler-arm.h
@@ -250,7 +250,7 @@ class MacroAssembler: public Assembler {
   // |object| is the object being stored into, |value| is the object being
   // stored.  value and scratch registers are clobbered by the operation.
   // The offset is the offset from the start of the object, not the offset from
-  // the tagged HeapObject pointer.  For use with FieldOperand(reg, off).
+  // the tagged HeapObject pointer.  For use with FieldMemOperand(reg, off).
   void RecordWriteField(
       Register object,
       int offset,
@@ -1532,7 +1532,7 @@ class CodePatcher {
 // -----------------------------------------------------------------------------
 // Static helper functions.
 
-inline MemOperand ContextOperand(Register context, int index) {
+inline MemOperand ContextOperand(Register context, int index = 0) {
   return MemOperand(context, Context::SlotOffset(index));
 }