[v8][wasm] Add histograms for code gc

This adds two histograms to measure the amount of code freed by code GC.
Both expire after ~1 year, when also the other wasm histograms expire,
so we can review them all together for extension.
The histograms were added to v8 in https://crrev.com/c/1573698.

R=titzer@chromium.org, isherman@chromium.org

Bug: v8:8217
Change-Id: Ie2e51f484e9fb367623bce6102e8edd19ac6c853
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1579079
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653932}
1 file changed