Fix sampling of V8.DetachedContextAgeInGC histogram.
Fix sampling of V8.DetachedContextAgeInGC histogram.

BUG=

Review URL: https://codereview.chromium.org/938693002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26719}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh