Cancel SRIVerifier when the corresponding context is destroyed

Otherwise, DidGetReadable() is called with null ExecutionContext.

Bug: 758883
Change-Id: I904d0c946a1be928c1fe2bebb37efec384a7bd59
Reviewed-on: https://chromium-review.googlesource.com/655807
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501166}

third_party/WebKit/Source/modules/fetch/FetchManager.cpp

@@ -195,6 +195,11 @@ class FetchManager::Loader final
       reader_ = handle_->ObtainReader(this);
     }
 
+    void Cancel() {
+      reader_ = nullptr;
+      handle_ = nullptr;
+    }
+
     void DidGetReadable() override {
       DCHECK(reader_);
       DCHECK(loader_);
@@ -648,6 +653,8 @@ void FetchManager::Loader::Dispose() {
     loader_->Cancel();
     loader_ = nullptr;
   }
+  if (integrity_verifier_)
+    integrity_verifier_->Cancel();
   execution_context_ = nullptr;
 }