This CL marks v8::Isolate::GetEnteredContext as deprecated in favor of
replacing it with GetEnteredOrMicrotaskContext. Blink no longer uses it,
and Node.js does not use this too.

GetEnteredOrMicrotaskContext() is relevant for all known cases over
GetEnteredContext(), and it costs 2% of a benchmark score to maintain
the entered contexts under the nestable microtask context.
https://crrev.com/c/1322290 is a context for the bencmark and nestable
microtask contexts.

Bug: v8:8124
Change-Id: I260e32daadf34dc587926a1e20ab950ff2e31699
Reviewed-on: https://chromium-review.googlesource.com/c/1353025
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57927}