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This adds NavigationHandle::SetReferer() and calls it from
WebLayer's Navigation::SetRequestHeader().

BUG=1114870
TEST=covered by test

Change-Id: I5154e477422cccebad2a2a9af734e5401ce66533
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350384
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797792}