Fix PPB_MouseLock.LockMouse crash and add tests.

ppapi/thunk/enter.cc

Index: ppapi/thunk/enter.cc
diff --git a/ppapi/thunk/enter.cc b/ppapi/thunk/enter.cc
index dcfe2aa7f0545e1c2675864557a34efa3cb8c52e..e76e32fe517130e1e4f4b40f9045552b327d9bb9 100644
--- a/ppapi/thunk/enter.cc
+++ b/ppapi/thunk/enter.cc
@@ -237,6 +237,17 @@ EnterInstanceNoLock::EnterInstanceNoLock(PP_Instance instance)
   SetStateForFunctionError(instance, functions_, true);
 }
 
+EnterInstanceNoLock::EnterInstanceNoLock(
+    PP_Instance instance,
+    const PP_CompletionCallback& callback)
+    : EnterBase(0 /* resource */, callback),
+    // TODO(dmichael): This means that the callback_ we get is not associated
+    //                 even with the instance, but we should handle that for
+    //                 MouseLock (maybe others?).
+      functions_(PpapiGlobals::Get()->GetInstanceAPI(instance)) {
+  SetStateForFunctionError(instance, functions_, true);
+}
+
 EnterInstanceNoLock::~EnterInstanceNoLock() {
 }