PL_DHashTableLookup() had the same return protocol as SearchTable(), which
meant that compilers could generate a tail call from the former to the latter.
Bug 1124973 replaced PL_DHashTableLookup() with PL_DHashTableSearch(), which
has a slightly different return protocol, and so the tail call was lost. This
appears to be the cause of the Fennec performance regression.
This patch splits SearchTable() in two (using templates to avoid explicit code
duplication). SearchTable<false>() now has the same return protocol as
PL_DHashTableSearch(), and so the tail call can now be used again.
27f5668308