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Retrieving an ExecutionContext from the LocalFrame passed from
NavigatorClipboard ensures that the correct ExecutionContext for the current
clipboard operation is always used.

In cross-frame circumstances, ScriptState's ExecutionContext would be that
of the parent frame, but LocalFrame's ExecutionContext would be that of the
child frame.

This was mentioned in [1], and initially introduced in [2].

This should ensure that the incorrect (parent frame's) PermissionManager isn't
requested when determining whether a permission has been granted, or when
asking for permission.

This CL does not fix the potential for a null dereference if the
ExecutionContext is detached, as both the ExecutionContext::From and
GetExecutionContext will result in a null dereference in this case.

[1]: https://crrev.com/c/2016200/19/third_party/blink/renderer/modules/clipboard/clipboard_promise.cc#91
[2]: https://crrev.com/c/804973

Bug: 1002794
Change-Id: I340224959fb98d3112464f4955c6d88e537f6199
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057006
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746506}