Stop counting kOffMainThreadFetch in ResourceFetcher

OffMainThreadFetch has been stable for a long time, so let's remove it.

Bug: 443374
Change-Id: If9e8f3f1059c121e2c94cf2ee6da91581e7016e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1616827
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661280}
5 files changed