[wasm] grow_memory(0) is the same as memory_size()

BUG=chromium:653264
TEST=cctest/test-run-wasm-module/GrowMemoryZero
R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2389263005
Cr-Commit-Position: refs/heads/master@{#40030}

src/wasm/wasm-module.cc

@@ -1822,6 +1822,9 @@ int32_t GetInstanceMemorySize(Isolate* isolate, Handle<JSObject> instance) {
 
 int32_t GrowInstanceMemory(Isolate* isolate, Handle<JSObject> instance,
                            uint32_t pages) {
+  if (pages == 0) {
+    return GetInstanceMemorySize(isolate, instance);
+  }
   Address old_mem_start = nullptr;
   uint32_t old_size = 0, new_size = 0;
 

test/cctest/wasm/test-run-wasm-module.cc

@@ -283,6 +283,21 @@ TEST(Run_WasmModule_MemSize_GrowMem) {
   TestModule(&zone, builder, kExpectedValue);
 }
 
+TEST(GrowMemoryZero) {
+  // Initial memory size is 16, see wasm-module-builder.cc
+  static const int kExpectedValue = 16;
+  TestSignatures sigs;
+  v8::internal::AccountingAllocator allocator;
+  Zone zone(&allocator);
+
+  WasmModuleBuilder* builder = new (&zone) WasmModuleBuilder(&zone);
+  WasmFunctionBuilder* f = builder->AddFunction(sigs.i_v());
+  ExportAsMain(f);
+  byte code[] = {WASM_GROW_MEMORY(WASM_I32V(0))};
+  f->EmitCode(code, sizeof(code));
+  TestModule(&zone, builder, kExpectedValue);
+}
+
 TEST(Run_WasmModule_GrowMemoryInIf) {
   TestSignatures sigs;
   v8::internal::AccountingAllocator allocator;