Bug 1602444 - Prevent attaching twice to content pages from the browser toolbox. r=jdescottes

We were breaking twice in the browser toolbox because we were attaching to
all the content process targets as well as all frame targets.
But as frames (i.e. web pages) are running within the content processes,
we were having two thread actor attached to the same thread.
This is a stopgap solution for the browser toolbox and we would need
to do something better for the content toolboxes.

Differential Revision: https://phabricator.services.mozilla.com/D57505

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extra : moz-landing-system : lando