Remove unused InspectorFrontend declarations.

There seems to be nothing called InspectorFrontend. Removed unused
forward declarations for it and changed comments to say "frontend"
instead.

R=pfeldman@chromium.org,dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2046433002
Cr-Commit-Position: refs/heads/master@{#398080}
7 files changed