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Similar to functions, API events can be restricted on a per-context
basis. Apply the same deletion logic to them if they are unavailable
in the given context.

Modify APIBinding unittests to reflect this, and add more comprehensive
tests in NativeExtensionBindingsSystem to test APIs that can be
partially-available to websites.

BUG=653596

Review-Url: https://codereview.chromium.org/2821793003
Cr-Commit-Position: refs/heads/master@{#465736}