Promptly let go of WebURLLoader objects.

The objects keeping their own WebURLLoaders (and thus being
WebURLLoaderClients) must promptly let go of their ownership upon
becoming garbage. Not doing so risks the embedder calling the client
while it is in a sweepable state.

R=
BUG=568173

Review URL: https://codereview.chromium.org/1635113002

Cr-Commit-Position: refs/heads/master@{#371639}

third_party/WebKit/Source/core/fetch/ResourceLoader.h

@@ -51,6 +51,9 @@ public:
     ~ResourceLoader() override;
     DECLARE_TRACE();
 
+    // Promptly release m_loader.
+    EAGERLY_FINALIZE();
+
     void start();
     void changeToSynchronous();
 

third_party/WebKit/Source/core/loader/PingLoader.h

@@ -75,6 +75,9 @@ public:
 
     DECLARE_VIRTUAL_TRACE();
 
+    // Promptly finalize m_loader.
+    EAGERLY_FINALIZE();
+
 protected:
     PingLoader(LocalFrame*, ResourceRequest&, const FetchInitiatorInfo&, StoredCredentials);