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

src/arm/full-codegen-arm.cc

Index: src/arm/full-codegen-arm.cc
diff --git a/src/arm/full-codegen-arm.cc b/src/arm/full-codegen-arm.cc
index 7cb402e636b25903a995206b9cc48f1d82ef144c..b0df8461094e133dd5dd1e9df50801da478d2de6 100644
--- a/src/arm/full-codegen-arm.cc
+++ b/src/arm/full-codegen-arm.cc
@@ -165,14 +165,18 @@ void FullCodeGenerator::Generate() {
   int locals_count = info->scope()->num_stack_slots();
 
   info->set_prologue_offset(masm_->pc_offset());
-  // The following four instructions must remain together and unmodified for
-  // code aging to work properly.
-  __ stm(db_w, sp, r1.bit() | cp.bit() | fp.bit() | lr.bit());
-  // Load undefined value here, so the value is ready for the loop
-  // below.
-  __ LoadRoot(ip, Heap::kUndefinedValueRootIndex);
-  // Adjust fp to point to caller's fp.
-  __ add(fp, sp, Operand(2 * kPointerSize));
+  {
+    PredictableCodeSizeScope predictible_code_size_scope(
+        masm_, kNoCodeAgeSequenceLength * Assembler::kInstrSize);
+    // The following three instructions must remain together and unmodified
+    // for code aging to work properly.
+    __ stm(db_w, sp, r1.bit() | cp.bit() | fp.bit() | lr.bit());
+    // Load undefined value here, so the value is ready for the loop
+    // below.
+    __ LoadRoot(ip, Heap::kUndefinedValueRootIndex);
+    // Adjust FP to point to saved FP.
+    __ add(fp, sp, Operand(2 * kPointerSize));
+  }
 
   { Comment cmnt(masm_, "[ Allocate locals");
     for (int i = 0; i < locals_count; i++) {