Don't send any preferences that have a string value that is longer
than MAX_ADVISABLE_PREF_LENGTH. This is intended to mitigate OOM
issues, as I've seen a parent process crash trying to create a 100mb
message to send to the child. Such users likely cannot use e10s at
all.
This has a test for all combinations of setting the default and user
values of a preference to large or small string values, or not setting
them at all.
I manually verified that filtering out preferences reduces the size of
the IPC::Message that is sent to the child by printing out the size of
the reply message in PContentParent::OnMessageReceived().
Andrew McCreight