`Sec-Metadata`: `cause` is navigation-only.

This patch removes the `cause` member from `Sec-Metadata` headers
generated for non-navigation requests.

Spec: https://mikewest.github.io/sec-metadata/#cause-member

Bug: 843478
Change-Id: I1cc35909ecdccd7ac0f4ab440be899acf67a0181
Reviewed-on: https://chromium-review.googlesource.com/1100820Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Commit-Queue: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567571}

third_party/WebKit/LayoutTests/external/wpt/fetch/sec-metadata/fetch.tentative.https.sub.html

@@ -8,7 +8,7 @@
         .then(r => r.json())
         .then(j => {
           assert_header_equals(j.header, {
-            "cause": "forced",
+            "cause": undefined,
             "destination": "\"\"",
             "target": "subresource",
             "site": "same-origin"
@@ -21,7 +21,7 @@
         .then(r => r.json())
         .then(j => {
           assert_header_equals(j.header, {
-            "cause": "forced",
+            "cause": undefined,
             "destination": "\"\"",
             "target": "subresource",
             "site": "same-site"
@@ -34,7 +34,7 @@
         .then(r => r.json())
         .then(j => {
           assert_header_equals(j.header, {
-            "cause": "forced",
+            "cause": undefined,
             "destination": "\"\"",
             "target": "subresource",
             "site": "cross-site"

third_party/WebKit/LayoutTests/external/wpt/fetch/sec-metadata/img.tentative.https.sub.html

@@ -12,7 +12,7 @@
       "https://{{host}}:{{ports[https][0]}}/referrer-policy/generic/subresource/image.py",
       t.step_func_done(img => {
         assert_header_equals(decodeImageData(extractImageData(img)).headers["sec-metadata"], {
-          "cause": "forced",
+          "cause": undefined,
           "destination": "image",
           "target": "subresource",
           "site": "same-origin"
@@ -27,7 +27,7 @@
       "https://{{hosts[][www]}}:{{ports[https][0]}}/referrer-policy/generic/subresource/image.py",
       t.step_func_done(img => {
         assert_header_equals(decodeImageData(extractImageData(img)).headers["sec-metadata"], {
-          "cause": "forced",
+          "cause": undefined,
           "destination": "image",
           "target": "subresource",
           "site": "same-site"
@@ -42,7 +42,7 @@
       "https://{{hosts[alt][www]}}:{{ports[https][0]}}/referrer-policy/generic/subresource/image.py",
       t.step_func_done(img => {
         assert_header_equals(decodeImageData(extractImageData(img)).headers["sec-metadata"], {
-          "cause": "forced",
+          "cause": undefined,
           "destination": "image",
           "target": "subresource",
           "site": "cross-site"

third_party/WebKit/LayoutTests/external/wpt/fetch/sec-metadata/script.tentative.https.sub.html

@@ -10,7 +10,7 @@
     t.add_cleanup(_ => header = null);
 
     assert_header_equals(header, {
-      "cause": "forced",
+      "cause": undefined,
       "destination": "script",
       "target": "subresource",
       "site": "same-origin"
@@ -25,7 +25,7 @@
     t.add_cleanup(_ => header = null);
 
     assert_header_equals(header, {
-      "cause": "forced",
+      "cause": undefined,
       "destination": "script",
       "target": "subresource",
       "site": "same-site"
@@ -40,7 +40,7 @@
     t.add_cleanup(_ => header = null);
 
     assert_header_equals(header, {
-      "cause": "forced",
+      "cause": undefined,
       "destination": "script",
       "target": "subresource",
       "site": "cross-site"

third_party/blink/renderer/core/loader/base_fetch_context.cc

@@ -131,9 +131,8 @@ void BaseFetchContext::AddAdditionalRequestHeaders(ResourceRequest& request,
         }
       }
 
-      String value = String::Format(
-          "cause=%s, destination=%s, target=subresource, site=%s",
-          request.HasUserGesture() ? "user-activation" : "forced",
+      String value =
+          String::Format("destination=%s, target=subresource, site=%s",
                          destination_value, site_value);
       request.AddHTTPHeaderField("Sec-Metadata", AtomicString(value));
     }