Blink uses Isolate::GetEnteredContext() to implement HTML's "entry
context" concept, and thus depends on it not being changed except
explicitly (by Blink.) To support this, stop entering contexts
implicitly in all external API entry points; rather just set the
context as current. The only thing that changes the entered context
is now Context::Enter()/Context::Exit() (and Context::Scope.)

BUG=v8:6307

Review-Url: https://codereview.chromium.org/2862483003
Cr-Commit-Position: refs/heads/master@{#45064}