Make FetchAPIRequest::headers use case insensitive Compare

This is a prerequisite step for using FetchAPIRequest instead of
ServiceWorkerFetchRequest. ServiceWorkerFetchRequest has a member |headers| with
CaseInsensitiveCompare, but FetchAPIRequest has a member |headers| with the
default Compare function, so that this will cause behavior changes. This CL is
to change FetchAPIRequest::headers to use CaseInsensitiveCompare.

Bug: 908344, 789854
Change-Id: I8e1736d5c5ab2d291ff7c126afa0f4025983b84d
Reviewed-on: https://chromium-review.googlesource.com/c/1356463
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613074}
16 files changed