We have already parsed a function when we call CollectSourcePositions, so we
shouldn't update the parsing statistics again otherwise we will double-count.
Also, CollectSourcePositions needs to be made native-context independent, and
Blink's CountUsage counter requires a context to have been entered when it is
called and so isn't context independent.

BUG=chromium:992063

Change-Id: Idda50b98a8308f022cb90e1a18afb43982e95298
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1746472
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63151}