Annotate Heap::FindAllocationMemento for MemorySanitizer.

This function may intentionally, safely use uninitialized memory.

BUG=chromium:413232
LOG=N
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/585643002

Patch from Sergey Matveev <earthdok@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24081 ce2b1a6d-e550-0410-aec6-3dcde31c8c00