Unlike other v8::Context getter on v8::Isolate, the v8::Context returned
by GetEnteredOrMicrotaskContext may be non-NativeContext. However,
Blink implicitly assumes all v8::Context is backed by NativeContexts.

This CL updates GetEnteredOrMicrotaskContext() to check the resulting
Context is NativeContext, and updates callers of EnterMicrotaskContext
to use NativeContext.

Change-Id: Ifae528f4ce8feb067f4ad1a43330dc55f4a8ed79
Reviewed-on: https://chromium-review.googlesource.com/c/1301653Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57259}