Add a feature to disable fetch-keepalive

We're planning to use it to measure the memory impact of the feature.

Bug: 1000994
Change-Id: I64406765ab57e11aeb92bead2c50298fecca85db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787029
Commit-Queue: Yutaka Hirano <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Cr-Commit-Position: refs/heads/master@{#693651}
11 files changed