Fix the heap-buffer-overflow in linux_asan_chrome_mp build.
Fix the heap-buffer-overflow in linux_asan_chrome_mp build.

It seems in AddressSanitizer(Asan), there is a possibility that a word can be comprised of partial
combining characters or partial surrogate pair. In this case FindValidBoundaryBefore() / FindValidBoundaryAfter()
might return an index that doesn't lie within the segment range, which causes the heap-buffer-overflow error.

BUG=516361
TEST=No crash when trying to open fuzz-142.pdf on Asan chrome build

Review URL: https://codereview.chromium.org/1262273003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#343115}

ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend,CapOM/ChromiumGStreamerBackend,ltilve/ChromiumGStreamerBackend