Add base::Feature for TransferableStreams

Make it possible to enable TransferableStreams from the server side.
This will be useful if we find that we need it before the next branch
point.

Tested manually.

BUG=894838

Change-Id: I93eebe5f36c4a1277492febe1138503c040ac269
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2366953Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Auto-Submit: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800160}

content/child/runtime_features.cc

@@ -414,6 +414,8 @@ void SetRuntimeFeaturesFromChromiumFeatures() {
           {"RawClipboard", blink::features::kRawClipboard, kEnableOnly},
           {"ShadowDOMV0", blink::features::kWebComponentsV0, kUseFeatureState},
           {"StorageAccessAPI", blink::features::kStorageAccessAPI, kEnableOnly},
+          {"TransferableStreams", blink::features::kTransferableStreams,
+           kEnableOnly},
           {"TrustedDOMTypes", features::kTrustedDOMTypes, kEnableOnly},
           {"UserAgentClientHint", features::kUserAgentClientHint,
            kUseFeatureState},

testing/variations/fieldtrial_testing_config.json

@@ -6850,6 +6850,27 @@
             ]
         }
     ],
+    "TransferableStreams": [
+        {
+            "platforms": [
+                "android",
+                "android_weblayer",
+                "android_webview",
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "TransferableStreams"
+                    ]
+                }
+            ]
+        }
+    ],
     "TranslateCompactUI": [
         {
             "platforms": [

third_party/blink/common/features.cc

@@ -704,5 +704,10 @@ const base::Feature kReducedReferrerGranularity{
 const base::Feature kContentCaptureUserActivatedDelay = {
     "ContentCaptureUserActivatedDelay", base::FEATURE_DISABLED_BY_DEFAULT};
 
+// Enable ReadableStream, WritableStream and TransformStream objects to be
+// transferred with postMessage().
+const base::Feature kTransferableStreams{"TransferableStreams",
+                                         base::FEATURE_DISABLED_BY_DEFAULT};
+
 }  // namespace features
 }  // namespace blink

third_party/blink/public/common/features.h

@@ -284,6 +284,8 @@ BLINK_COMMON_EXPORT extern const base::Feature kReducedReferrerGranularity;
 BLINK_COMMON_EXPORT extern const base::Feature
     kContentCaptureUserActivatedDelay;
 
+BLINK_COMMON_EXPORT extern const base::Feature kTransferableStreams;
+
 }  // namespace features
 }  // namespace blink