Fix potential overwriting of debug jumps of following code.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 29817a3161dd3afe6922ccfa0d5503de622eff2f..07e05614ef915a8e92eebf32898bb655628a7b5f 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -553,8 +553,9 @@ class Assembler : public Malloced {
   // TODO(X64): Rename this, removing the "Real", after changing the above.
   static const int kRealPatchReturnSequenceAddressOffset = 2;
 
-  // The x64 JS return sequence is padded with int3 to make it large
-  // enough to hold a call instruction when the debugger patches it.
+  // Some x64 JS code is padded with int3 to make it large
+  // enough to hold an instruction when the debugger patches it.
+  static const int kJumpInstructionLength = 13;
   static const int kCallInstructionLength = 13;
   static const int kJSReturnSequenceLength = 13;
 
@@ -583,6 +584,8 @@ class Assembler : public Malloced {
   // Some mnemonics, such as "and", are the same as C++ keywords.
   // Naming conflicts with C++ keywords are resolved by adding a trailing '_'.
 
+  // Inserts nop instructions taking up n bytes.
+  void Pad(int n);
   // Insert the smallest number of nop instructions
   // possible to align the pc offset to a multiple
   // of m. m must be a power of 2.
@@ -1111,6 +1114,12 @@ class Assembler : public Malloced {
   void call(Label* L);
   void call(Handle<Code> target, RelocInfo::Mode rmode);
 
+  // Calls directly to the given address using a relative offset.
+  // Should only ever be used in Code objects for calls within the
+  // same Code object. Should not be used when generating new code (use labels),
+  // but only when patching existing code.
+  void call(Address target);
+
   // Call near absolute indirect, address in register
   void call(Register adr);