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The in-construction bit is considered an implementation detail of the
garbage collector and should not be relied on by user objects.

ActiveScriptWrappable objects are queried by the garbage collector for
pending activities and rooted as long as there are activities pending.

Since the GC may interrupt construction of ASW objects the virtual
methods for HasPendingActivity() and potentially GetExecutionContext()
may not be called.

While ASW(Base) HasPendingActivity() may provide a default body for
HasPendingActivity() and thus work around the construction problem,
the call to GetExecutionContext() is dispatched through CRTP and there
are three different use cases:
a) GetExecutionContext() may still be pure virtual at ASW
registration (EventTarget).
b) GetExecutionContext() may already be marked as final at ASW
registration (Node)
c) GetExecutionContext() may be a regular method (users of
ExecutionContextClient)

Use case a) would require a default body for GetExecutionContext in
ASW while use case b) prohibits that. The other alternative would have
been adding
  virtual ScriptWrappable::GetExecutionContext() { return nullptr; }

Since ExecutionContext is not a concept of platform layering would be
violated. (Even though just a forward declaration would suffice.)

Bug: chromium:1056170
Change-Id: Ibf3cfb600eaa7f3e11e76579268b52689883440c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2167433
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762847}