Added Sec-Purpose header to prefetch requests.

This CL adds the "Sec-Purpose" header which is added to any prefetch
request. Direct prefetch requests that do not use the prefetch proxy
will have a value of "prefetch", and prefetch requests that do use the
prefetch proxy will have a value of "prefetch;anonymous-client-ip". This
will allow servers to differentiate between the two types of prefetch
requests.

Bug: 1290561
Change-Id: I24928fd75b6a55f92109022e94beeca54b53e2b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3421706
Reviewed-by: Robert Ogden <[email protected]>
Commit-Queue: Max Curran <[email protected]>
Cr-Commit-Position: refs/heads/main@{#965381}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 6cc814c..3911d9c7 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -1258,6 +1258,8 @@
     "prefetch/no_state_prefetch/no_state_prefetch_tab_helper.h",
     "prefetch/pref_names.cc",
     "prefetch/pref_names.h",
+    "prefetch/prefetch_headers.cc",
+    "prefetch/prefetch_headers.h",
     "prefetch/prefetch_prefs.cc",
     "prefetch/prefetch_prefs.h",
     "prefetch/prefetch_proxy/chrome_speculation_host_delegate.cc",