Only one FrameInspector can be active at a time on any given stack,
this ensures that it's lifetime is sufficiently scoped.

R=yangguo@chromium.org
TEST=mjsunit/regress/regress-crbug-259300

Review URL: https://codereview.chromium.org/1034743002

Cr-Commit-Position: refs/heads/master@{#27477}