[UserTimingL3] Turn on L3 behaviour by default.

To get L3 behaviour before, users would have to pass a special
command-line flag, --enable-blink-features=CustomUserTiming.

This changes makes L3 the default behaviour. The intent-to-ship for L3
can be found here:
https://groups.google.com/a/chromium.org/d/topic/blink-dev/CWN34Qo8icI/discussion

Users can opt-out of L3 by passing
--disable-blink-features=CustomUserTiming.

Bug: 758385
Change-Id: I316b2a929fc0748c3b9a9c1ef3d4c551ae00eafa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758715Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Reviewed-by: Nicolás Peña Moreno <npm@chromium.org>
Commit-Queue: Tom McKee <tommckee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692060}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -487,7 +487,7 @@
     },
     {
       name: "CustomUserTiming",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "Database",

third_party/blink/web_tests/virtual/stable/http/tests/serviceworker/webexposed/global-interface-listing-service-worker-expected.txt

@@ -919,9 +919,11 @@ interface PerformanceEntry
     method toJSON
 interface PerformanceMark : PerformanceEntry
     attribute @@toStringTag
+    getter detail
     method constructor
 interface PerformanceMeasure : PerformanceEntry
     attribute @@toStringTag
+    getter detail
     method constructor
 interface PerformanceObserver
     static getter supportedEntryTypes

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-dedicated-worker-expected.txt

@@ -858,9 +858,11 @@ Starting worker: resources/global-interface-listing-worker.js
 [Worker]     method toJSON
 [Worker] interface PerformanceMark : PerformanceEntry
 [Worker]     attribute @@toStringTag
+[Worker]     getter detail
 [Worker]     method constructor
 [Worker] interface PerformanceMeasure : PerformanceEntry
 [Worker]     attribute @@toStringTag
+[Worker]     getter detail
 [Worker]     method constructor
 [Worker] interface PerformanceObserver
 [Worker]     static getter supportedEntryTypes

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -4725,9 +4725,11 @@ interface PerformanceLongTaskTiming : PerformanceEntry
     method toJSON
 interface PerformanceMark : PerformanceEntry
     attribute @@toStringTag
+    getter detail
     method constructor
 interface PerformanceMeasure : PerformanceEntry
     attribute @@toStringTag
+    getter detail
     method constructor
 interface PerformanceNavigation
     attribute @@toStringTag

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-shared-worker-expected.txt

@@ -853,9 +853,11 @@ Starting worker: resources/global-interface-listing-worker.js
 [Worker]     method toJSON
 [Worker] interface PerformanceMark : PerformanceEntry
 [Worker]     attribute @@toStringTag
+[Worker]     getter detail
 [Worker]     method constructor
 [Worker] interface PerformanceMeasure : PerformanceEntry
 [Worker]     attribute @@toStringTag
+[Worker]     getter detail
 [Worker]     method constructor
 [Worker] interface PerformanceObserver
 [Worker]     static getter supportedEntryTypes