[FlexNG] Fix finch flag

Turns out we've not been getting perf data or a 50/50 experiment.

I verified it this time by running

out/Release/content_shell --enable-features=FlexNG

both before and after this change. Before: legacy flex. After: FlexNG.

Bug: 845235
Change-Id: Ie053bd9a8e188f8f42b540f3c1f8e03627714f64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148655Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759003}

content/child/runtime_features.cc

@@ -351,7 +351,7 @@ void SetRuntimeFeaturesFromChromiumFeatures() {
            kEnableOnly},
           {"FeaturePolicyForClientHints",
            features::kFeaturePolicyForClientHints, kUseFeatureState},
-          {"FlexNG", blink::features::kFlexNG, kEnableOnly},
+          {"LayoutNGFlexBox", blink::features::kFlexNG, kEnableOnly},
           {"FontAccess", blink::features::kFontAccess, kUseFeatureState},
           {"FontSrcLocalMatching", features::kFontSrcLocalMatching,
            kUseFeatureState},