Supplement should have a Member to the corresponding Supplementable object

third_party/WebKit/Source/platform/Supplementable.h

Index: third_party/WebKit/Source/platform/Supplementable.h
diff --git a/third_party/WebKit/Source/platform/Supplementable.h b/third_party/WebKit/Source/platform/Supplementable.h
index c9ff4c197dab5b62421b72066915b50e2b85a205..c2b90df0f0a3ec2457dac03425f1d030a2dfbfe9 100644
--- a/third_party/WebKit/Source/platform/Supplementable.h
+++ b/third_party/WebKit/Source/platform/Supplementable.h
@@ -95,6 +95,12 @@ class Supplementable;
 template <typename T>
 class Supplement : public GarbageCollectedMixin {
  public:
+  // TODO(haraken): Remove the default constructor.

[sof, 2017/01/19 07:38:39]

I'd argue that this isn't a natural goal for all supplements -- there are some
implementations where the extension/supplement effectively only wants to be
anonymously hosted somewhere once instantiated.

Which allows them to be a declared supplement of several potential hosts, but
only register with one (e.g., LocalFileSystem, ImageBitmapFactories). Recasting
their implementations to be single-hosted could be done, but the result feels
cumbersome.

(https://codereview.chromium.org/2645613004/ is a trial example how it comes out
for LocalFileSystem.)

+  // All Supplement objects should be instantiated with m_host.
+  Supplement() {}
+  explicit Supplement(T& host) : m_host(&host) {}
+  T* host() const { return m_host; }
+
   static void provideTo(Supplementable<T>& host,
                         const char* key,
                         Supplement<T>* supplement) {
@@ -109,7 +115,10 @@ class Supplement : public GarbageCollectedMixin {
     return host ? host->requireSupplement(key) : 0;
   }
 
-  DEFINE_INLINE_VIRTUAL_TRACE() {}
+  DEFINE_INLINE_VIRTUAL_TRACE() { visitor->trace(m_host); }
+
+ private:
+  Member<T> m_host;
 };
 
 // Supplementable<T> inherits from GarbageCollectedMixin virtually