Simplify and speed up address-to-page cache for conservative stack scanning.

This change makes the per-heap address-to-page cache positive only and handles
large pages more naturally. There is now a separate whole-process negative
cache that keeps track of pages that are not in any of the heaps. This speeds up my microbenchmark from ~87 to ~65.

R=ager@chromium.org, haraken@chromium.org, wibling@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/271703002

git-svn-id: svn://svn.chromium.org/blink/trunk@173749 bbb929c8-8fbe-4397-9dbb-9b2b20218538