MIPS: Simplify pending message script handling.

Port f71e2626

Original commit message:
This removes the separate tracking of the pending message script,
because that script is already stored in the message object and
duplicating it in the ThreadLocalTop makes it more brittle.

BUG=

Review URL: https://codereview.chromium.org/999763003

Cr-Commit-Position: refs/heads/master@{#27140}

src/mips/full-codegen-mips.cc

@@ -5336,24 +5336,12 @@ void FullCodeGenerator::EnterFinallyBlock() {
   __ lw(a1, MemOperand(at));
   __ SmiTag(a1);
   __ push(a1);
-
-  ExternalReference pending_message_script =
-      ExternalReference::address_of_pending_message_script(isolate());
-  __ li(at, Operand(pending_message_script));
-  __ lw(a1, MemOperand(at));
-  __ push(a1);
 }
 
 
 void FullCodeGenerator::ExitFinallyBlock() {
   DCHECK(!result_register().is(a1));
   // Restore pending message from stack.
-  __ pop(a1);
-  ExternalReference pending_message_script =
-      ExternalReference::address_of_pending_message_script(isolate());
-  __ li(at, Operand(pending_message_script));
-  __ sw(a1, MemOperand(at));
-
   __ pop(a1);
   __ SmiUntag(a1);
   ExternalReference has_pending_message =

src/mips64/full-codegen-mips64.cc

@@ -5338,24 +5338,12 @@ void FullCodeGenerator::EnterFinallyBlock() {
   __ ld(a1, MemOperand(at));
   __ SmiTag(a1);
   __ push(a1);
-
-  ExternalReference pending_message_script =
-      ExternalReference::address_of_pending_message_script(isolate());
-  __ li(at, Operand(pending_message_script));
-  __ ld(a1, MemOperand(at));
-  __ push(a1);
 }
 
 
 void FullCodeGenerator::ExitFinallyBlock() {
   DCHECK(!result_register().is(a1));
   // Restore pending message from stack.
-  __ pop(a1);
-  ExternalReference pending_message_script =
-      ExternalReference::address_of_pending_message_script(isolate());
-  __ li(at, Operand(pending_message_script));
-  __ sd(a1, MemOperand(at));
-
   __ pop(a1);
   __ SmiUntag(a1);
   ExternalReference has_pending_message =