[wasm] Initialize the root register for WASM tests.

src/compiler/linkage.h

Index: src/compiler/linkage.h
diff --git a/src/compiler/linkage.h b/src/compiler/linkage.h
index 33da7a4c2b6b4dfcbffb2f34eeb7ce83ec594e64..564f779e9b5b9f601233e36ec71d7a1f799afb9d 100644
--- a/src/compiler/linkage.h
+++ b/src/compiler/linkage.h
@@ -157,6 +157,8 @@ class CallDescriptor final : public ZoneObject {
     kUseNativeStack = 1u << 7,
     // (arm64 only) call instruction has to restore JSSP.
     kRestoreJSSP = 1u << 8,
+    // Causes the code generator to initialize the root register.
+    kInitializeRootRegister = 1u << 9,
     kPatchableCallSiteWithNop = kPatchableCallSite | kNeedsNopAfterCall
   };
   typedef base::Flags<Flag> Flags;
@@ -223,6 +225,9 @@ class CallDescriptor final : public ZoneObject {
   bool NeedsFrameState() const { return flags() & kNeedsFrameState; }
   bool SupportsTailCalls() const { return flags() & kSupportsTailCalls; }
   bool UseNativeStack() const { return flags() & kUseNativeStack; }
+  bool InitializeRootRegister() const {
+    return flags() & kInitializeRootRegister;
+  }
 
   LinkageLocation GetReturnLocation(size_t index) const {
     return location_sig_->GetReturn(index);
@@ -327,8 +332,9 @@ class Linkage : public ZoneObject {
   // for the host platform. This simplified calling convention only supports
   // integers and pointers of one word size each, i.e. no floating point,
   // structs, pointers to members, etc.
-  static CallDescriptor* GetSimplifiedCDescriptor(Zone* zone,
-                                                  const MachineSignature* sig);
+  static CallDescriptor* GetSimplifiedCDescriptor(
+      Zone* zone, const MachineSignature* sig,
+      bool set_initialize_root_flag = false);
 
   // Creates a call descriptor for interpreter handler code stubs. These are not
   // intended to be called directly but are instead dispatched to by the