Set a maximum size on favicons

Favicons are downloaded by the render process and sent back via mojo message.
However, mojo messages have a maximum size; if the favicon doesn't fit, the
render process is killed. Impose a maximum size to ensure that favicons can fit
in mojo messages.

Task-number: QTBUG-69123
Change-Id: I6027bc310dcc77ec530145e4f125efa36106464f
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>