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There once was a ScriptExecutionObserver::Delegate, but it disappeared
in some past refactoring. The lifetime around ScriptExecutionObserver
is quite complex, and relies on base::ObserverList being a
SupportsWeakPtr, which we want to stop doing for https://crbug.com/888973.

There is now exactly one ScriptExecutionObserver implementation
(ActivityLog), and the observer list only ever has exactly 1 or zero
observers in it; added and removed in concert with a TabHelper.
WebViewGuest also has an ObserverList<ScriptExecutionObserver>, but
it never added any observers to it.

We can instead make ActivityLog responsible for the lifetime of its
callbacks from ScriptExecutor. It already dispenses WeakPtrs to itself.

Bug: 888973
Change-Id: Ic773bbcbabea70627b48ef8e0e29ff4e16d1e1f5
Reviewed-on: https://chromium-review.googlesource.com/c/1256398
Commit-Queue: Trent Apted <tapted@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596454}