This CL adds a new event that enables the back-end to send coverage
updates on its own initiative. This event can be triggered via the C++
method `triggerPreciseCoverageDeltaUpdate` on the agent in a way that
causes coverage data to be immediatelly collected.

This is useful in the back-end to collect coverage at a certain point
in time, i.e. when a lifecycle event such as first contentful paint
occurs.

The previous interface could not support this, because it could not
reasonably be triggered from C++, and if triggered through the protocol,
dispatching messages added delay that invalidated the data (i.e. data
might have been taken too late to be accurate).

TBR=yangguo@chromium.org

Change-Id: I0f7201412a8d64866e6e314e5bc850354c13a9da
Bug: chromium:1022031
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1992437
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65864}