MIPS[64] Fix SizeOfInitialHeap test failing

Commit 0d833cb9 deleted function call
that created CEntryStub code in snapshot. So when CEntryStub::GetCode is
called while initializing V8, the code for it, is not found in shapshot.
Hence this code has to be generated in new allocation on heap, that is
why initial heap is above the limit.
This patch assures that CEntryStub code is in the snapshot.

Bug: 
Change-Id: Ifc885998e9253ab3e19e4e2409e4c124a5f1b0d1
Reviewed-on: https://chromium-review.googlesource.com/593950Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@imgtec.com>
Cr-Commit-Position: refs/heads/master@{#47028}

src/mips/code-stubs-mips.cc

@@ -881,6 +881,8 @@ void CodeStub::GenerateFPStubs(Isolate* isolate) {
 void CEntryStub::GenerateAheadOfTime(Isolate* isolate) {
   CEntryStub stub(isolate, 1, kDontSaveFPRegs);
   stub.GetCode();
+  CEntryStub save_doubles(isolate, 1, kSaveFPRegs);
+  save_doubles.GetCode();
 }
 
 

src/mips64/code-stubs-mips64.cc

@@ -878,6 +878,8 @@ void CodeStub::GenerateFPStubs(Isolate* isolate) {
 void CEntryStub::GenerateAheadOfTime(Isolate* isolate) {
   CEntryStub stub(isolate, 1, kDontSaveFPRegs);
   stub.GetCode();
+  CEntryStub save_doubles(isolate, 1, kSaveFPRegs);
+  save_doubles.GetCode();
 }