[inspector] revert fallThrough support for async methods

We can't use "this" after calling agent code since agent code can destroy dispatcher.

BUG=chromium:668358
TBR=dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2526183002
Cr-Commit-Position: refs/heads/master@{#41247}

third_party/inspector_protocol/README.v8

@@ -14,3 +14,4 @@ description.
 Local modifications:
 - This only includes the lib/ and templates/ directories, scripts, build
   and the LICENSE files.
+- FallbackThrough support is removed for async methods.

third_party/inspector_protocol/templates/TypeBuilder_cpp.template

@@ -367,7 +367,7 @@ DispatchResponse::Status DispatcherImpl::{{command.name}}(int callId, std::uniqu
       {%- endfor -%}
         {%- if command.parameters -%}, {% endif -%}
         std::move(callback));
-    return lastCallbackFallThrough() ? DispatchResponse::kFallThrough : DispatchResponse::kAsync;
+    return DispatchResponse::kAsync;
     {% endif %}
 }
   {% endfor %}