Handle segfaults while walking native heap

Vendor blobs on ryu mprotect heap pages, causing segfaults when dumping
unreachable memory.  Handle segfaults within HeapWalker by mapping a
zero page over any unreadable pages.  HeapWalker runs in the forked
process, so the mapping will not affect the original process.

Bug: 28269332
Change-Id: I16245af722123f2ad467cbc6f245a70666c55544