Disable BlinkGenPropertyTrees by default

Due to existing regressions, this patch disables BlinkGenPropertyTrees.

For completeness (to prevent another http://crbug.com/935195), tested:
+------------------------------------------------+-----------+------------+
|                      flag                      | pre-patch | post-patch |
+------------------------------------------------+-----------+------------+
|                     no flag                    | BGPT on   |  BGPT on*  |
| --enable-features=BlinkGenPropertyTrees        | BGPT on   |  BGPT on   |
| --disable-features=BlinkGenPropertyTrees       | BGPT off  |  BGPT off  |
| --enable-blink-features=BlinkGenPropertyTrees  | BGPT on   |  BPGT on   |
| --disable-blink-features=BlinkGenPropertyTrees | BGPT off  |  BGPT off  |
+------------------------------------------------+-----------+------------+
* This is due to the finch field trial testing config.

content/child/runtime_features.cc now checks for experimental web platform
features because the BlinkGenPropertyTrees flag is fully controlled by the
feature default in third_party/blink/common/features.cc and would be disabled
in web tests (formerly layout tests) without this check.

Bug: 836884
Change-Id: I590e1b7c697c7c70cf3c4c5121483b2dd76c74df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1505836Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638706}

content/child/runtime_features.cc

@@ -227,7 +227,8 @@ void SetIndividualRuntimeFeatures(
 
   WebRuntimeFeatures::EnableFeatureFromString(
       "BlinkGenPropertyTrees",
-      base::FeatureList::IsEnabled(blink::features::kBlinkGenPropertyTrees));
+      base::FeatureList::IsEnabled(blink::features::kBlinkGenPropertyTrees) ||
+          enable_experimental_web_platform_features);
 
   WebRuntimeFeatures::EnablePassiveDocumentEventListeners(
       base::FeatureList::IsEnabled(features::kPassiveDocumentEventListeners));

third_party/blink/common/features.cc

@@ -42,7 +42,7 @@ const base::Feature kJankTrackingSweepLine{"JankTrackingSweepLine",
 // Enable a new compositing mode called BlinkGenPropertyTrees where Blink
 // generates the compositor property trees. See: https://crbug.com/836884.
 const base::Feature kBlinkGenPropertyTrees{"BlinkGenPropertyTrees",
-                                           base::FEATURE_ENABLED_BY_DEFAULT};
+                                           base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Enable LayoutNG.
 const base::Feature kLayoutNG{"LayoutNG", base::FEATURE_DISABLED_BY_DEFAULT};

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -155,7 +155,7 @@
     },
     {
       name: "BlinkGenPropertyTrees",
-      status: "stable",
+      status: "experimental",
     },
     {
       name: "BlinkRuntimeCallStats",