Annotate mapped memory regions for LeakSanitizer.

Annotate mapped memory regions for LeakSanitizer.

Add in-code annotations for LeakSanitizer to treat any memory allocated through
the VirtualMemory class as a source of live pointers. This change eliminates
false positive leak reports when running Chromium under LSan.

BUG=chromium:328552
R=mstarzinger@chromium.org
LOG=Y

Review URL: https://codereview.chromium.org/137313002

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

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18589 ce2b1a6d-e550-0410-aec6-3dcde31c8c00