ARM: make MacroAssembler::LoadRoot emit exactly one instruction when in predictable code size scope.

src/arm/codegen-arm.cc

Index: src/arm/codegen-arm.cc
diff --git a/src/arm/codegen-arm.cc b/src/arm/codegen-arm.cc
index 047003531b35c2098d6230786dba3a4165b1cc06..776cbfb55c4b14671adf1562cbc72427c3069590 100644
--- a/src/arm/codegen-arm.cc
+++ b/src/arm/codegen-arm.cc
@@ -606,6 +606,7 @@ static byte* GetNoCodeAgeSequence(uint32_t* length) {
   *length = kNoCodeAgeSequenceLength * Assembler::kInstrSize;
   if (!initialized) {
     CodePatcher patcher(byte_sequence, kNoCodeAgeSequenceLength);
+    PredictableCodeSizeScope scope(patcher.masm(), *length);
     patcher.masm()->stm(db_w, sp, r1.bit() | cp.bit() | fp.bit() | lr.bit());
     patcher.masm()->LoadRoot(ip, Heap::kUndefinedValueRootIndex);
     patcher.masm()->add(fp, sp, Operand(2 * kPointerSize));