[FlexNG] Enable FlexNG by default

Change the blink runtime flag to be fully controlled by the finch flag
so that we can still use finch as a kill switch if necessary. (With the
old kEnableOnly, "stable" in runtime enabled features json would
override the finch server's value.)

Bug: 845235
Change-Id: I7610b078a52674b3e8fa06d87d33f82e96761d35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216595Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772090}

content/child/runtime_features.cc

@@ -353,7 +353,7 @@ void SetRuntimeFeaturesFromChromiumFeatures() {
            blink::features::kIgnoreCrossOriginWindowWhenNamedAccessOnWindow,
            kEnableOnly},
           {"LayoutNG", blink::features::kLayoutNG, kUseFeatureState},
-          {"LayoutNGFlexBox", blink::features::kFlexNG, kEnableOnly},
+          {"LayoutNGFlexBox", blink::features::kFlexNG, kUseFeatureState},
           {"LegacyWindowsDWriteFontFallback",
            features::kLegacyWindowsDWriteFontFallback, kUseFeatureState},
           {"OriginPolicy", features::kOriginPolicy, kUseFeatureState},

third_party/blink/common/features.cc

@@ -490,7 +490,7 @@ const base::FeatureParam<int> kFontPreloadingDelaysRenderingParam{
     &kFontPreloadingDelaysRendering, "delay-in-ms", 100};
 
 const base::Feature kFlexGaps{"FlexGaps", base::FEATURE_DISABLED_BY_DEFAULT};
-const base::Feature kFlexNG{"FlexNG", base::FEATURE_DISABLED_BY_DEFAULT};
+const base::Feature kFlexNG{"FlexNG", base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kKeepScriptResourceAlive{"KeepScriptResourceAlive",
                                              base::FEATURE_DISABLED_BY_DEFAULT};

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -928,7 +928,6 @@
       // provides a convenient way for testing legacy layout code path in blink
       // unit tests.
       name: "LayoutNG",
-      // Keep this list in sync with the one in LayoutNGFlexBox below.
       implied_by: ["LayoutNGBlockFragmentation", "LayoutNGFieldset", "LayoutNGFragmentItem", "LayoutNGGrid", "EditingNG", "BidiCaretAffinity", "LayoutNGTable", "LayoutNGFragmentTraversal"],
       status: "stable",
     },
@@ -940,8 +939,7 @@
     },
     {
       name: "LayoutNGFlexBox",
-      implied_by: ["LayoutNGBlockFragmentation", "LayoutNGFieldset", "LayoutNGFragmentItem", "LayoutNGGrid", "EditingNG", "BidiCaretAffinity", "LayoutNGTable", "LayoutNGFragmentTraversal"],
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "LayoutNGForControls",