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This CL finishes moving the feature flags for Isolated Prerender to a
separate header file.

I decided not to add <Feature>IsEnabled() helpers to the params file
since it doesn't seem that much better because tests still need to
reference the feature directly anyways. Maybe reconsider if the feature
logic gets any more complex in the future.

Bug: 1023483
Change-Id: Ic139ed099a44736644c7f5414dfd4bb0e622b077
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2021311Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735198}