src: ignore unused warning for inspector-agent.cc

Currently the following compiler warning is displayed:

  ../src/inspector_agent.cc:218:5: warning: ignoring return value of
  function declared with warn_unused_result attribute [-Wunused-result]
      callback->Call(env_->context(), receiver, 1, &argument);
      ^~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 warning generated.

This commit does a static cast of the result as there are tests that
fail if we try to do something like ToLocalChecked.

PR-URL: https://github.com/nodejs/node/pull/13188


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>