[url_launcher][web] Prevent browser from navigating when followLink isn't called

[mdebbar]

When a DOM click event is received, we need to make a decision before the end of the event loop whether to allow the browser to navigate or not. At the end of the event loop, the browser will perform the navigation if we don't prevent it.

The problem occurs when the followLink signal arrives AFTER the event loop (this can happen in semantics mode when the web engine uses a debouncer to queue events and send them to the framework after some delay). This leads to a situation where we can't make a definitive decision by the end of the event loop, so this PR does the following:

1. [best case] If the followLink signal is received before the end of the event loop, we let the browser do the navigation for a full web link experience.

2. [meh case] If no followLink signal is received before the end of the event loop, we PREVENT the browser from navigating. But we keep waiting for the followLink signal. If the signal arrives a bit later, we fallback to a programmatic navigation through the launchUrl API.

Fixes flutter/flutter#162927
Fixes flutter/flutter#162408

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