Add comments about the reason of keeping the LayoutNG runtime flag

Change-Id: I2fb6037a3bed30a8d1d43c3f997471c695461e4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2053616Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741088}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -897,6 +897,11 @@
       status: "experimental",
     },
     {
+      // LayoutNG has been enabled in M76, but we still keep this flag for
+      // testing. See web_tests/FlagExpectations/disable-layout-ng for more
+      // details about running web tests with LayoutNG disabled. This flag also
+      // provides a convenient way for testing legacy layout code path in blink
+      // unit tests.
       name: "LayoutNG",
       implied_by: ["LayoutNGBlockFragmentation", "LayoutNGFieldset", "LayoutNGFlexBox", "LayoutNGFragmentItem", "LayoutNGLineCache", "EditingNG", "BidiCaretAffinity", "LayoutNGTable", "LayoutNGFragmentTraversal"],
       status: "stable",

third_party/blink/web_tests/FlagExpectations/disable-layout-ng

@@ -3,6 +3,14 @@
 # tags: [ Release Debug ]
 # results: [ Timeout Crash Pass Failure Slow Skip ]
 
+# These expectations are used when LayoutNG is disabled. We test this
+# configuration on trybot linux_layout_tests_layout_ng_disabled.
+# Though LayoutNG has been enabled in M76, we still keep running web tests with
+# LayoutNG disabled because we still use legacy layout for editing, multicol,
+# etc. Though we have tests for editing and multicol, the tests don't fully
+# cover all layout code paths, so we need to run all web tests to ensure legacy
+# layout still fully works.
+
 # Tests that fail in legacy but pass in NG
 
 # accessibility/