Simplify TryCatch::CanContinue() checks

This CL assumes that:

  If `TryCatch::CanContinue()` is false for a `TryCatch`,
  `TryCatch::CanContinue()` is false for all outer `TryCatch`es.

At least this holds for forcible worker termination.
(test: https://chromium-review.googlesource.com/c/chromium/src/+/2355160)

This CL refactors the code based on this assumption.
This will make upcoming refactoring simpler.

Bug: 1111134
Change-Id: I041209b2bcc8cd28112987a5a97f55e3fa180c71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2409481
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808218}

third_party/blink/renderer/bindings/core/v8/worker_or_worklet_script_controller.cc

@@ -368,12 +368,11 @@ ClassicEvaluationResult WorkerOrWorkletScriptController::EvaluateAndReturnValue(
     // script evaluation code paths.
     if (!block.CanContinue()) {
       ForbidExecution();
-    }
-
-    if (IsExecutionForbidden()) {
       return ClassicEvaluationResult();
     }
 
+    CHECK(!IsExecutionForbidden());
+
     if (!block.HasCaught()) {
       // Step 10. If evaluationStatus is a normal completion, then return
       // evaluationStatus. [spec text]