Missing return_rw_data calls in LayoutTask?

[Ms2ger]

Some RPC queries (in particular, hit_test/get_nodes_under_mouse) are made from script without requesting a reflow first. In order to prevent these queries finishing before the first reflow, we lock the LayoutTaskData in LayoutTask::start.

We still want to allow layout code to update the LayoutTaskData before the first reflow, so we allow it to call LayoutTask::lock_rw_data to take the lock, and then calling LayoutTask::return_rw_data to return the lock (keeping the script code waiting).

However, some callers (at least GetCurrentEpoch, GetWebFontLoadState and CollectReports) don't call LayoutTask::return_rw_data, and thus leave the script thread free to read from the LayoutTaskData.

As far as I can tell, there is no fundamental reason none of those can happen before the first reflow.

[Ms2ger]

CC @pcwalton