Add supported checks to the SVG library, and library cleanup.

tools/dom/scripts/systemnative.py

Index: tools/dom/scripts/systemnative.py
diff --git a/tools/dom/scripts/systemnative.py b/tools/dom/scripts/systemnative.py
index c82e8618cb1f4a739016ab86e7cd2d0315032e57..e293dd0e01fc56d603c82aa017e0e7c56cc290b3 100644
--- a/tools/dom/scripts/systemnative.py
+++ b/tools/dom/scripts/systemnative.py
@@ -235,7 +235,11 @@ class DartiumBackend(HtmlDartGenerator):
 
   def GetSupportCheck(self):
     # Assume that everything is supported on Dartium.
-    return 'true'
+    value = js_support_checks.get(self._interface.doc_js_name)
+    if type(value) == tuple:
+      return (value[0], 'true')
+    else:
+      return 'true'
 
   def FinishInterface(self):
     self._GenerateCPPHeader()