[Backport] CVE-2021-21207: Use after free in IndexedDB

Manual backport and adaptation to BindingSet of patch originally reviewed on
https://chromium-review.googlesource.com/c/chromium/src/+/2778871:
Never fail in ReceiverSet::Add

Because of how UniqueReceiverSet is implemented and used, it is
dangerous to allow Add() to fail: callers reasonably assume that added
objects are still alive immediately after the Add() call.

This changes ReceiverId to a uint64 and simply CHECK-fails on
insert collision.

This fundamentally increases binary size of 32-bit builds, because
a widely used 32-bit data type is expanding to 64 bits for the sake
of security and stability. It is effectively unavoidable for now, and
also just barely above the tolerable threshold.

A follow-up (but less backwards-mergeable) change should be able to
reduce binary size beyond this increase by consolidating shared
code among ReceiverSet template instantiations.

Fixed: 1185732
Change-Id: I9acf6aaaa36e10fdce5aa49a890173caddc13c52
Binary-Size: Unavoidable (see above)
Commit-Queue: Ken Rockot <rockot@google.com>
Auto-Submit: Ken Rockot <rockot@google.com>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#865815}
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

Author: krockot

chromium/mojo/public/cpp/bindings/binding_set.h

@@ -35,7 +35,7 @@ struct BindingSetTraits<Binding<Interface, ImplRefTraits>> {
   }
 };
 
-using BindingId = size_t;
+using BindingId = uint64_t;
 
 template <typename ContextType>
 struct BindingSetContextTraits {
@@ -279,10 +279,10 @@ class BindingSetBase {
                            RequestType request,
                            Context context) {
     BindingId id = next_binding_id_++;
-    DCHECK_GE(next_binding_id_, 0u);
     auto entry = std::make_unique<Entry>(std::move(impl), std::move(request),
                                          this, id, std::move(context));
-    bindings_.insert(std::make_pair(id, std::move(entry)));
+    auto result = bindings_.insert(std::make_pair(id, std::move(entry)));
+    CHECK(result.second) << "BindingId overflow with collision";
     return id;
   }