Also adds a NullContextScope for code which wants to ensure it is context
independent. Removes a workaround in V8ProfilerAgentImpl::startProfiling
which created a context due to CollectSourcePositions not being context
indpendent.

BUG=chromium:992063

Change-Id: I94c7eea6416dc64bc61fb8ff9cd945449a791a77
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1748693
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63176}