blink_gc_plugin: detect singletons with embedded ScriptWrappables.

tools/clang/blink_gc_plugin/tests/static_singleton.txt

Index: tools/clang/blink_gc_plugin/tests/static_singleton.txt
diff --git a/tools/clang/blink_gc_plugin/tests/static_singleton.txt b/tools/clang/blink_gc_plugin/tests/static_singleton.txt
new file mode 100644
index 0000000000000000000000000000000000000000..c591c711bb9e181cd5f1a4d006044821993b96ef
--- /dev/null
+++ b/tools/clang/blink_gc_plugin/tests/static_singleton.txt
@@ -0,0 +1,31 @@
+static_singleton.cpp:14:3: warning: [blink-gc] The static singleton 'object' contains one or more unsafe ScriptWrappable references
+  DEFINE_STATIC_LOCAL(HeapObject, object);
+  ^
+static_singleton.cpp:8:3: note: expanded from macro 'DEFINE_STATIC_LOCAL'
+  BLINK_STATIC_SINGLETON()              \
+  ^
+./heap/stubs.h:192:3: note: expanded from macro 'BLINK_STATIC_SINGLETON'
+  __attribute__((annotate("blink_gc_singleton_type")))
+  ^
+note: [blink-gc] Embedded unsafe ScriptWrappable-derived field with type 'class blink::AScriptWrappable' located at 'b_ => a_'
+static_singleton.cpp:22:5: warning: [blink-gc] The static singleton 'object' contains one or more unsafe ScriptWrappable references
+    DEFINE_STATIC_LOCAL(HeapObject, object);
+    ^
+static_singleton.cpp:8:3: note: expanded from macro 'DEFINE_STATIC_LOCAL'
+  BLINK_STATIC_SINGLETON()              \
+  ^
+./heap/stubs.h:192:3: note: expanded from macro 'BLINK_STATIC_SINGLETON'
+  __attribute__((annotate("blink_gc_singleton_type")))
+  ^
+note: [blink-gc] Embedded unsafe ScriptWrappable-derived field with type 'class blink::AScriptWrappable' located at 'b_ => a_'
+static_singleton.cpp:25:3: warning: [blink-gc] The static singleton 'object' contains one or more unsafe ScriptWrappable references
+  DEFINE_STATIC_LOCAL(HeapObject, object);
+  ^
+static_singleton.cpp:8:3: note: expanded from macro 'DEFINE_STATIC_LOCAL'
+  BLINK_STATIC_SINGLETON()              \
+  ^
+./heap/stubs.h:192:3: note: expanded from macro 'BLINK_STATIC_SINGLETON'
+  __attribute__((annotate("blink_gc_singleton_type")))
+  ^
+note: [blink-gc] Embedded unsafe ScriptWrappable-derived field with type 'class blink::AScriptWrappable' located at 'b_ => a_'
+3 warnings generated.