Let the Leak Sanitizer ignore PageMemory objects

The LSan runs before the shutdown sequence runs. PageMemory is released in the shutdown sequence. As a result, the PageMemory is reported as leaking in LSan.

To suppress the report, this CL adds an annotation to let LSan ignore PageMemory objects.

See the bug description for more details.

BUG=369429

Review URL: https://codereview.chromium.org/264393002

git-svn-id: svn://svn.chromium.org/blink/trunk@173508 bbb929c8-8fbe-4397-9dbb-9b2b20218538