Remove WebContentsObserver::DidGetUserGesture.

This CL removes DidGetUserGesture and its various hooks in content/. All
existing clients of this method have been migrated to
WebContentsObserver::DidGetUserInteraction.

This is a follow up to https://crrev.com/1388293002, which implemented
WebContentsObserver::DidGetUserInteraction. Compared to
DidGetUserGesture, DidGetUserInteraction has a simpler implementation in
content/, and signals to clients the type of user interaction which
triggered it. DidGetUserGesture was intended to signal the types of
gestures which would indicate that the user was attempting to trigger an
action (Enter, Space, mouse click, gesture tap). However, gesture
signals are always noisy; the download request limiter, external
protocol handler, and resource dispatch host (which consumed
DidGetUserGesture) treated the gesture as a heuristic, rather than as
a canonical indication of a user's intent. In general, it is impossible
to know whether any of the user inputs were actually intended to trigger
an action. This makes DidGetUserGesture suitable *only* as a heuristic.

DidGetUserInteraction is intended as a complete replacement for
DidGetUserGesture, except it triggers on all keypress events rather than
only Enter and Space. This expansion is appropriate given the
aforementioned heuristic-nature of this signal.

BUG=584154

Review URL: https://codereview.chromium.org/1735833002

Cr-Commit-Position: refs/heads/master@{#378071}

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