Re-enable LayoutViewIsolationNodes.

This patch re-enables isolation nodes for layout views (iframes).

It seems that this causes a memory regression on one of the telemetry
tests, but it seems that it might be a timing issue. The evidence for
this is that the regression happens in free memory (evictable) and
the tile sizes seem to be same or similar with or without the flag.

For more details, see the referenced bug.

R=chrishtr@chromium.org, pdr@chromium.org, wangxianzhu@chromium.org

Bug: 896328
Change-Id: I9c399f253f246364b68980350cec3d14e5690b75
Reviewed-on: https://chromium-review.googlesource.com/c/1292511Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601301}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -676,6 +676,7 @@
     },
     {
       name: "LayoutViewIsolationNodes",
+      status: "stable",
     },
     {
       name: "LazyFrameLoading",