to IsGeneratingEmbeddedBuiltins() to clarify its meaning.

Bug: v8:6666
Change-Id: I8b282f29775a103a03f502c3e9629b40b4a690bd
Reviewed-on: https://chromium-review.googlesource.com/c/1480380Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59815}