Promote CompositeSVG to stable status

Bug: 1101002
Change-Id: I326d9949765f6f821502e439f0b2bfc7156023c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2496186Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821093}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -350,7 +350,7 @@
     },
     {
       name: "CompositeSVG",
-      status: "experimental",
+      status: "stable"
     },
     {
       name: "CompositingOptimizations",