GitLab

This is the first patch for the off-main-thread fetch project.
https://goo.gl/hgmQbP

We will implement this under "--enable-features=OffMainThreadFetch" flag.

I will submit https://codereview.chromium.org/2799883002, when it will be ready to test in Chrome.

BUG=443374

Review-Url: https://codereview.chromium.org/2798083004
Cr-Commit-Position: refs/heads/master@{#462822}