Enable WebXR Multi GPU support by default

Enables WebXR's makeXRCompatible API to support machines with more than
one GPU. The feature is controlled by the
blink::features::kWebXrMultiGpu flag. The WebXRMultiGpu Blink runtime
enabled features was added only to enable the feature flag in tests.

Bug: 792657,876140
Change-Id: If3a2e71a58da98066b1171d8ca54ef07bc2946fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2503840Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Patrick To <patrto@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#823946}

third_party/blink/common/features.cc

@@ -712,7 +712,7 @@ const char kSkipTouchEventFilterFilteringProcessParamValueBrowserAndRenderer[] =
 
 // Improves support for WebXR on computers with multiple GPUs.
 const base::Feature kWebXrMultiGpu{"WebXRMultiGpu",
-                                   base::FEATURE_DISABLED_BY_DEFAULT};
+                                   base::FEATURE_ENABLED_BY_DEFAULT};
 
 // Enables dependency support in blink::MatchedPropertiesCache, which allows
 // caching of previously uncachable objects.

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -2213,7 +2213,7 @@
       name: "WebXRMultiGpu",
       depends_on: ["WebXR"],
       // Enabled when blink::features::kWebXrMultiGpu is enabled.
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "WebXRPlaneDetection",