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Reland the change to extra header handling with the actual behaviour
change behind a disabled-by-default base::Feature. UMA metrics are
collected even when the feature is disabled, simulating what would have
happened had the feature been enabled, so we can better gauge the
compatibility impact on applications.

Bug: 1038002
Change-Id: I3d722060cc4526a14adac076bbf4eec0a0992b70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2229420
Commit-Queue: Richard Coles <torne@chromium.org>
Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775115}