Remove slow path branch

The fast paths aren't pure and have side effects like calling out to
the debugger and runtime calls. Note: These aren't "fast" paths per se,
but just *native promise* code paths.

Forcing the slow path omits these calls to the debugger and runtime
causing test failures.

Bug: v8:7148
Change-Id: Idf46a33622a6edf03d69fefa4c6bfb7efc8ea625
Reviewed-on: https://chromium-review.googlesource.com/824102Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50197}

src/builtins/builtins-promise-gen.cc

@@ -366,8 +366,6 @@ Node* PromiseBuiltinsAssembler::InternalPromiseThen(Node* context,
   VARIABLE(var_deferred_on_resolve, MachineRepresentation::kTagged);
   VARIABLE(var_deferred_on_reject, MachineRepresentation::kTagged);
 
-  GotoIfForceSlowPath(&promise_capability);
-
   Branch(WordEqual(promise_fun, constructor), &fast_promise_capability,
          &promise_capability);
 
@@ -1489,8 +1487,6 @@ TF_BUILTIN(PromiseReject, PromiseBuiltinsAssembler) {
   Label if_nativepromise(this), if_custompromise(this, Label::kDeferred);
   Node* const native_context = LoadNativeContext(context);
 
-  GotoIfForceSlowPath(&if_custompromise);
-
   Node* const promise_fun =
       LoadContextElement(native_context, Context::PROMISE_FUNCTION_INDEX);
   Branch(WordEqual(promise_fun, receiver), &if_nativepromise,