Fix PagedMemory, use -1 FD for mmap to fix WASM

PagedMemory was mistakenly passing 0 as FD to mmap().
It doesn't make a difference on Linux/Mac because the implementation
ignores the FD when MAP_ANONYMOUS is specified. It is failing however
on WASM, because the syscall handler looks at the FD first.
fd=-1 is also what man mmap suggests to use.

Also fix the output of trace_to_systrace to emit more frequent
status updates on WASM. This is to avoid the UI going quiet for
too long.

Change-Id: I4b0f7d77f3773b7eafef7576b2037a93a2417626