We might have to collect source positions for existing functions when we
start profiling, and doing so requires a context to be entered in V8. Ensure
we always enter a context by creating a temporary context and entering it before
starting profiling.

BUG=chromium:992063

Change-Id: I72b259a3ad31b712b0475f7729fb4ffdab5cdd96
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1745481
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63149}