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
(cherry picked from commit ba5d9ff6d9)