Currently, the external API (e.g. v8::Object::Get()) will enter the
context passed to it automatically. This is incorrect and causes some
trouble for Blink, so we want to change that.

It then becomes a potential problem to call the external API without
first entering a context, which the inspector code does in some
places. This patch aims to correct this.

BUG=v8:6307

Review-Url: https://codereview.chromium.org/2841053002
Cr-Commit-Position: refs/heads/master@{#44917}