Instead of directly using v8_enable_inspector_override from
build_overrides/v8.gni in all the GN configs, set a v8_enable_inspector
variable based on v8_enable_inspector_override and use that everywhere.
This is the more common pattern seen in over projects, and reduces the
need to include //build_overrides/v8.gni in many files.

Review-Url: https://codereview.chromium.org/2520683002
Cr-Commit-Position: refs/heads/master@{#41156}