Consider the following situation (one among many possible).
- FrameA has an oopif child FrameB;
- FrameA navigates to same-process origin (e.g. about:blank);
- at the same time, FrameC is attached to the FrameB in the
FrameB's process.
In this case, we get `frameNavigated` event for FrameA, immediately
followed by `frameAttached` event for FrameC. Since we detach all
FrameA's child frames on navigation, including the oopif FrameB,
there is no parent frame for FrameC to attach to.
In general, multiple processes coming from oopif may send their
events in wildly different order, and their view about the frame
tree may not always correspond to the "up to date" frame tree as
seen from the main frame's process. We try to keep our frame tree
aligned with what main process thinks, and ignore events that
reference frames absent in this tree.
Drive-by: handle filechooser exceptions because of async processing.
We used to do fetch() to decode the file buffer. However, this is
blocked by strict CSP policy. Instead, we can use explicit
string -> bytes conversion, and trade performance for CSP compliance.
