Introduce a flag for cross-origin prefetch + NetworkIsolationKey

This CL introduces a flag specifically for NetworkIsolationKey changes
made to cross-origin main-resource prefetches. We'd like to implement
these changes incrementally and not have them enabled implicitly under
the SplitCacheByNetworkIsolationKey flag.

A future CL will actually put this flag to use, and remove more
instances of the SplitCache feature detection.

[email protected], [email protected]

Bug: 939317
Change-Id: Ifd465d343b76e6533fffed46cfae1d2486fb078f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775890
Commit-Queue: Dominic Farolino <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Cr-Commit-Position: refs/heads/master@{#692021}
7 files changed